Reading: Sentence Completion: IELTS Academic 9990 (IELTS Academic)

The duck-billed platypus, found only in the rivers and streams of eastern Australia, is one of just two mammal groups that lay eggs, yet its most extraordinary adaptation has nothing to do with reproduction. When a platypus dives to hunt along a riverbed, folds of skin seal shut over its eyes, ears and nostrils, leaving the animal effectively blind, deaf and unable to smell for the length of a typical dive, which lasts around ninety seconds. Deprived of its ordinary senses, the platypus instead relies almost entirely on its bill, a soft, flexible structure covered in roughly forty thousand tiny sensory receptors.

These receptors are of two distinct kinds. Mechanoreceptors detect the faint pressure waves created by a moving shrimp or insect larva pushing water aside, while a second, rarer type, electroreceptors, can sense the weak electrical field generated whenever a prey animal's muscles contract. By comparing the tiny delay between the mechanical and electrical signals reaching receptors on either side of its bill, the platypus can calculate not just the direction of its prey but roughly how far away it is, a technique researchers have compared to a simple form of biological triangulation.

Electroreception of this kind is rare among mammals; it is far more commonly associated with sharks and rays, whose electroreceptive organs, called ampullae of Lorenzini, evolved entirely independently. The platypus and its close relative the echidna appear to have developed the ability through a completely separate evolutionary pathway, which is why biologists regard the platypus bill as a striking case of convergent evolution rather than a shared inherited trait. Laboratory studies in the 1980s first confirmed the bill's electrical sensitivity by recording nerve responses to a weak artificial current, overturning the earlier assumption that the organ served only a mechanical function. Because a hunting platypus swims with its eyes closed, sweeping its bill from side to side in a continuous searching motion, researchers now describe the behaviour less as swimming than as a form of underwater scanning.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. While diving with its eyes, ears and nostrils sealed shut, a platypus hunts primarily using its ____________________.
2. A typical platypus hunting dive lasts about ____________________.
3. The platypus's bill is covered in around ____________________ tiny sensory receptors.
4. Receptors that detect pressure waves from a moving shrimp or insect larva are called ____________________.
5. Electroreceptors sense the weak electrical field produced when a prey animal's ____________________ contract.
6. In sharks and rays, the organs responsible for electroreception are called the ____________________ of Lorenzini.
7. Because the platypus and its relatives evolved electroreception separately from sharks and rays, biologists treat the platypus bill as an example of ____________________ evolution.
8. Nerve-response studies conducted in the ____________________ first confirmed that the platypus bill could detect electrical signals.

For centuries, agricultural researchers assumed that bees were the primary pollinators of most flowering crops, largely because bees are active during daylight hours and therefore easy to observe and count. Recent field studies using infrared time-lapse cameras have overturned this assumption, revealing that moths and other nocturnal insects visit flowers far more frequently after dark than had previously been recorded, and in some hedgerow habitats deliver a share of successful pollination roughly equal to that of daytime bees. This discovery has drawn fresh attention to a threat increasingly facing night-flying pollinators: artificial light at night, a phenomenon ecologists commonly refer to using the abbreviation ALAN, which covers the glow produced by streetlamps, illuminated signage and household security lighting.

Moths navigate partly by orienting themselves relative to the moon and stars, keeping these distant light sources at a constant angle as they fly in a straight line. A nearby streetlamp disrupts this system because the insect attempts to maintain the same constant angle to a light that, unlike the moon, is close enough for its position to shift noticeably as the moth moves past it; the result is the familiar spiralling flight that traps a moth circling a bulb for hours rather than foraging among flowers. Researchers studying hedgerows near artificial lighting found that moth activity on illuminated flowers fell by roughly one third compared with flowers in darker stretches of the same hedgerow, and that the pollination of certain plant species dependent on nocturnal visitors declined correspondingly.

In response, several conservation trials have tested amber-coloured LED streetlamps, which emit light at wavelengths less disruptive to insect vision than the blue-rich white LEDs increasingly common in street lighting. Early results from a two-year trial in rural Cornwall suggest that switching to amber lighting, combined with shielding fixtures so that light is directed downward rather than outward, can measurably reduce the disorientation effect while still meeting road-safety lighting standards.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. Bees were long assumed to be the main crop pollinators partly because they are active during ____________________.
2. Ecologists commonly use the abbreviation ____________________ to describe artificial light produced at night by sources such as streetlamps and signage.
3. When navigating by the moon or stars, a moth keeps the light source at a constant ____________________ as it flies.
4. Because a streetlamp is far closer than the moon, a moth trying to hold the same angle to it ends up performing the ____________________ that traps it near the bulb.
5. In the hedgerow study, moth activity on illuminated flowers fell by roughly ____________________ compared with flowers in darker areas.
6. Some conservation trials have tested replacing white streetlamps with ____________________ LED streetlamps.
7. The two-year lighting trial described in the passage took place in rural ____________________.

More than ninety-five percent of the world's international data traffic, everything from video calls to financial transactions, travels not through satellites, as many people assume, but through a network of submarine fibre-optic cables laid along the ocean floor. A single modern cable typically contains fewer than a dozen fibre-optic pairs, hair-thin strands of glass along which data travels as pulses of laser light rather than as an electrical signal, allowing capacity and speed far beyond what a satellite link, hampered by physical distance and the delay it introduces, can achieve.

Laying a submarine cable begins long before any ship reaches open water. Marine surveyors first spend months studying detailed seabed maps to plot a route that avoids fishing grounds, shipwrecks and, wherever possible, geologically unstable regions prone to underwater landslides. Specialised cable-laying vessels then feed the cable from enormous rotating drums on deck, lowering it to the seabed at a controlled rate; in shallower coastal waters, a plough towed behind the ship buries the cable roughly one to two metres beneath the seabed to protect it from fishing trawlers and ship anchors, the two most common causes of cable damage.

Despite this protection, faults remain a routine hazard: on average, a submarine cable somewhere in the world is damaged roughly every three days, most often by a dragged anchor or a trawler's fishing gear rather than by a shark bite, a cause that generates disproportionate media attention despite being genuinely rare. When a fault is detected, a repair ship sails to the affected coordinates, grapples the cable from the seabed using a hook lowered on a line, hauls the damaged section to the surface, and splices in a new length before lowering the repaired cable back down, a process that can take one to three weeks depending on weather and water depth.

Because so few cables exist relative to the volume of traffic they carry, a handful of narrow maritime corridors, among them the Luzon Strait and the Strait of Malacca, have become chokepoints where a large share of intercontinental data physically converges, meaning that a single incident in one of these corridors can measurably slow internet traffic across entire continents.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. More than ____________________ of the world's international data traffic travels through submarine cables rather than satellites.
2. Inside a fibre-optic pair, data travels as pulses of ____________________ rather than as an electrical signal.
3. Before a cable is laid, surveyors study seabed maps to avoid fishing grounds, shipwrecks and areas prone to underwater ____________________.
4. In shallow coastal waters, a plough buries the cable to protect it from fishing trawlers and ____________________.
5. On average, a submarine cable somewhere in the world is damaged roughly every ____________________.
6. Most cable faults are caused by dragged anchors or fishing gear rather than by a ____________________, a cause that receives disproportionate media attention.
7. After hauling the damaged section to the surface, a repair ship ____________________ in a new length of cable.
8. A cable repair can take one to three weeks depending on weather and ____________________.

Long before the telephone made instant communication commonplace, several major cities experimented with a different technology to speed up urgent written messages: networks of pneumatic tubes buried beneath the streets. The system relied on a simple principle: a cylindrical carrier containing a paper message was inserted into a tube, and compressed air, generated by steam-powered pumping stations, propelled the carrier through the network to its destination at speeds that could exceed thirty kilometres per hour.

Paris opened the most extensive of these networks in 1866, eventually linking hundreds of post offices across the city with tubes that carried a distinctive lightweight paper form known as a pneumatique, sold specifically for use in the system. Messages sent by pneumatique were popular with journalists, stockbrokers and anyone needing to relay time-sensitive information faster than a horse-drawn messenger could manage, and the network remained in continuous operation for over a century, finally closing in 1984 after telex machines and telephones had made it largely redundant.

New York City built a rival network primarily to move mail between post offices rather than private messages between individuals; its tubes, roughly twenty centimetres in diameter, could send a canister containing around six hundred letters between stations in a matter of minutes, a volume no team of human runners could match. Postal workers nicknamed the pressurised carriers rockets because of the distinctive hissing sound the compressed air made as a canister rushed through the tube.

Pneumatic networks eventually declined everywhere for broadly similar reasons: the underground tubes were expensive to maintain, prone to blockages when a canister jammed partway along a route, and increasingly outcompeted by electronic communication that required no physical object to be moved at all. A handful of surviving tube segments have since found new uses; several hospitals still use short pneumatic systems today to send blood samples and small packages quickly between departments, a modest echo of the elaborate citywide systems that once linked entire capitals.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. The Paris pneumatic tube network opened in ____________________.
2. Messages sent through the Paris system were written on a special lightweight paper form called a ____________________.
3. The Paris network finally closed in ____________________ after telex machines and telephones made it redundant.
4. New York's pneumatic tubes were roughly ____________________ in diameter.
5. A single canister in the New York system could carry around ____________________ letters between stations.
6. Postal workers nicknamed the pressurised canisters ____________________ because of the hissing sound made by the compressed air.
7. Today, some hospitals still use short pneumatic systems to send small packages and ____________________ quickly between departments.

Few sights unsettle hospital visitors more than a surgical ward where patients recovering from reattachment surgery are treated with live leeches, yet the practice, once dismissed as an outdated relic of pre-modern medicine, has quietly returned to specialised centres worldwide. Its modern use has nothing to do with the discredited historical theory of bad blood that drove leech bleeding treatments for centuries; instead, leeches are applied for a precise physiological reason connected to the biology of reattached tissue.

When a finger, ear or other body part is surgically reattached, surgeons can usually reconnect the small arteries that pump blood into the tissue, but the far more delicate veins responsible for draining blood back out are frequently too narrow or too damaged to reconnect successfully. Without adequate drainage, blood pools in the reattached tissue, and the resulting pressure can cut off circulation entirely, causing the graft to die within days. A feeding leech relieves this pressure directly: its bite releases hirudin, a natural anticoagulant compound in its saliva that prevents the blood it draws from clotting, allowing continuous drainage that lasts for several hours after the leech itself has detached and fallen away.

Medicinal leeches used in hospitals today are raised in sterile facilities and are never reused between patients, addressing one of the main hygiene objections raised when the therapy was first reintroduced during the 1980s. A typical treatment course applies leeches to the affected tissue several times a day for roughly a week, the period during which the reattached tissue's own tiny veins gradually grow large enough to handle drainage unaided.

Researchers have since isolated hirudin and produced it synthetically for use in other anticoagulant medications, yet synthetic hirudin alone cannot replicate the mechanical drainage a feeding leech provides, which is why live leeches, rather than a purely chemical alternative, remain standard practice in microsurgical recovery wards for tissue at genuine risk of venous congestion.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. The discredited historical theory that once justified leech bleeding treatments is known as the theory of ____________________.
2. After reattachment surgery, arteries can usually be reconnected, but the ____________________ responsible for draining blood are often too narrow to reconnect.
3. Without adequate drainage, rising pressure in the reattached tissue can cause the graft to ____________________ within days.
4. A leech's bite releases ____________________, an anticoagulant compound that stops the drawn blood from clotting.
5. Leech-assisted drainage can continue for several hours after the leech itself has ____________________ and fallen away.
6. Hospitals address hygiene concerns by raising leeches in sterile facilities and ensuring they are never ____________________ between patients.
7. Hygiene objections to leech therapy were first raised when it was reintroduced during the ____________________.
8. A typical treatment applies leeches several times a day for roughly ____________________, until the tissue's own veins grow large enough to drain unaided.

Psychologists who study self-control have become increasingly interested in a phenomenon known as decision fatigue: the observation that the quality of a person's choices tends to decline the more decisions that person has already made in a given period. The underlying idea is that deliberate, effortful choosing draws on a limited mental resource, and as that resource is used up, later decisions are made with less care, less analysis, or simply avoided altogether through impulsive selection of the easiest option.

Early laboratory demonstrations of this effect asked volunteers to make a long series of trivial choices, such as selecting between two consumer products, before then testing their self-control on an unrelated task, such as holding a hand in ice water for as long as possible. Volunteers who had made many prior choices consistently gave up sooner than volunteers who had instead spent an equivalent amount of time simply evaluating the same products without choosing between them. The researchers concluded that it was the act of choosing itself, rather than mere exposure to options, that consumed the limited resource.

Outside the laboratory, decision fatigue has been proposed as an explanation for a range of everyday patterns. Shoppers who must compare many similar items late in a long shopping trip often report simply grabbing a familiar brand rather than weighing the alternatives, and staff working in roles that require frequent case-by-case judgements (approving or declining routine applications, for instance) appear to apply stricter, more effortful scrutiny to cases reviewed early in a shift than to those reviewed near its end. Some organisations have responded by rearranging workflows so that the most consequential judgements are scheduled earlier, when decision-making capacity is presumed to be freshest, and by building in short breaks intended to allow the depleted resource to partially recover.

Critics caution that the size of the effect varies considerably between studies and that factors such as low blood glucose, general tiredness and boredom may all contribute to the same pattern of declining care, making it hard to isolate decision fatigue as a single, distinct cause.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. According to the passage, effortful choosing draws on a ______ mental resource.
2. In the early laboratory studies, volunteers first chose between two ______ before their self-control was tested.
3. Their self-control was then tested by asking them to hold a hand in ______ for as long as possible.
4. Volunteers who had already made many choices gave up ______ than those who had only evaluated the products.
5. Staff who make frequent case-by-case judgements often apply less rigorous scrutiny to cases reviewed near the end of a ______.
6. Critics point out that ______ such as low blood glucose may also help explain the same pattern of declining care.

A worker cooperative is a business that is owned and governed by the people who work in it, rather than by outside shareholders or a single proprietor. Instead of ownership being tied to invested capital, each worker-member typically holds one voting share regardless of role or seniority, and major decisions, such as electing a management board or approving the annual budget, are settled on a one-member, one-vote basis at a general meeting. Profits, once operating costs and any agreed reinvestment have been set aside, are usually distributed among members according to hours worked or length of service rather than in proportion to capital contributed.

Proponents argue that this ownership structure changes incentives in ways that benefit both workers and the wider business. Because members share directly in the profits they help generate, cooperatives are often reported to have lower staff turnover than comparable conventionally owned firms in the same sector, and members frequently describe a stronger sense of responsibility for the quality of their own work. Some research further suggests that worker cooperatives tend to weather economic downturns better than investor-owned competitors, since members facing a fall in demand are often willing to accept temporary reductions in hours or pay across the whole workforce rather than resort immediately to layoffs.

The model is not without practical difficulties, however. Raising start-up capital can be harder for a cooperative than for a conventional company, since banks and outside investors have little influence over decisions and therefore limited incentive to lend, leaving many cooperatives reliant on members' own savings or specialist ethical-finance lenders. Reaching agreement among a large membership can also slow decision-making considerably compared with a firm run by a single owner or a small board, particularly when a proposal is contentious. To address this, many larger cooperatives delegate day-to-day operational decisions to an elected management team while reserving only the most significant strategic choices, such as mergers or major capital investment, for a full membership vote.

Despite these constraints, the cooperative sector has continued to expand steadily in several economies, particularly within retail, agriculture and professional services, where the model's emphasis on shared ownership appears to appeal strongly to workers seeking greater control over their working conditions.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. In a worker cooperative, each member usually holds one ______ regardless of their job role.
2. Major decisions are typically approved on a one-member, one-vote basis at a ______.
3. Profits are generally shared out among members according to ______ or length of service.
4. Cooperatives are often reported to have lower ______ than comparable conventionally owned firms.
5. During downturns, members often accept temporary reductions in hours or pay rather than resorting immediately to ______.
6. Raising start-up capital can be difficult for cooperatives because banks have little influence over decisions and therefore limited ______ to lend.
7. Many larger cooperatives delegate day-to-day decisions to an elected ______ while reserving major choices for a full membership vote.

In most secondary school systems, students progress from one grade to the next primarily on the basis of age: a student who turns twelve in a given year is placed in the grade associated with that age, regardless of how thoroughly they mastered the material covered the year before. A small but growing number of schools have begun experimenting with an alternative model, often called competency-based or mastery-based progression, in which students advance through a course only once they have demonstrated a defined level of proficiency in its content, rather than automatically after a fixed number of weeks has elapsed.

Under a mastery-based model, a course is broken down into a sequence of discrete units, each accompanied by a clear description of what a student must do to be judged proficient. Students who do not meet the required standard on an initial assessment are not simply given a failing mark and moved on; instead, they receive targeted additional instruction and are permitted to reattempt the assessment until proficiency is demonstrated. Students who master a unit quickly, conversely, may move on immediately rather than waiting for the rest of their age cohort, which advocates argue removes the boredom that can arise when capable students are held back by a fixed pace.

Early adopters report several apparent benefits. Teachers often note that grades come to reflect what a student has actually learned rather than behaviour, attendance or effort, since a final mark is awarded only once genuine proficiency has been shown. Some schools also report a narrowing of the achievement gap between weaker and stronger students, attributing this to the additional instructional time made available to those who initially struggle.

The model nonetheless poses considerable logistical challenges. Because students no longer move through content at a uniform pace, a single classroom may contain students working on several different units simultaneously, placing heavy demands on teachers to prepare differentiated materials. Reporting progress to parents and to external qualification authorities, many of which still expect a single grade tied to a fixed academic year, can also prove awkward under a system designed to decouple learning from the calendar. For these reasons, most schools that have adopted mastery-based progression have so far applied it only within individual subjects rather than across an entire institution.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. In most schools, students move up a grade mainly according to their ______, rather than what they have learned.
2. Under mastery-based progression, students move on only after demonstrating a defined level of ______ in the course content.
3. Each unit includes a clear description of what a student must do to be judged ______.
4. Students who fail an initial assessment receive extra instruction and are allowed to ______ the assessment.
5. Advocates claim that letting quick learners move on immediately removes the ______ caused by a fixed pace.
6. Teachers report that grades come to reflect actual learning rather than behaviour, attendance or ______.
7. Some schools report a narrowing of the ______ between weaker and stronger students.
8. A single classroom may contain students working on several different units at once, requiring teachers to prepare ______.

Karst is the name given to a distinctive type of landscape that develops where a thick layer of soluble rock, most commonly limestone, lies close to the surface and is gradually dissolved by slightly acidic rainwater. Rain absorbs a small amount of carbon dioxide as it falls through the atmosphere and picks up more as it passes through soil, forming a weak solution of carbonic acid that reacts chemically with the calcium carbonate in limestone, slowly widening cracks in the rock over thousands of years.

As this process, known as chemical weathering, continues, surface streams that once flowed across the rock may disappear entirely, sinking underground through widened cracks called sinkholes and continuing their course as subterranean rivers. Over time, the enlarged cracks can join to form extensive networks of underground caves and passages, some large enough to be explored on foot, along which the dissolved water continues to carve new channels or deposit dissolved minerals as stalactites and stalagmites where it drips from a cave roof.

On the surface, karst regions are often recognisable from a distinctive lack of visible rivers and lakes, since most drainage has migrated underground, combined with a landscape pitted with numerous sinkholes and, in some regions, taller residual towers of unweathered rock left standing after the surrounding limestone has been dissolved away. Farming on karst terrain can be difficult, since soils are often thin and rainwater drains away too quickly to be retained near the surface for crops, though the underground cave networks themselves can support unusual, often highly localised ecosystems adapted to permanent darkness.

Karst landscapes also present distinctive hazards. Because so much of the rock beneath the surface has been dissolved away, the ground above an underground cavity can collapse suddenly if it becomes too weak to support its own weight, a hazard that engineers must account for carefully before approving construction in karst regions. Water flowing rapidly through underground channels is also difficult to filter naturally, so pollutants introduced at the surface can travel long distances underground and contaminate drinking water drawn from distant wells with little warning.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. Karst landscapes form where a layer of soluble rock, usually ______, lies close to the surface.
2. As it falls and passes through soil, rainwater becomes a weak solution of ______.
3. Surface streams may disappear underground through widened cracks called ______.
4. Dissolved minerals may be deposited as stalactites and ______ where water drips from a cave roof.
5. Farming on karst terrain is difficult partly because soils are thin and ______ drains away too quickly.
6. The ground above an underground cavity can collapse if it becomes too ______ to support its own weight.

Over the past several years, a growing number of small rural towns in scenic but economically peripheral regions have experienced a sudden reversal of a decades-long trend: rather than losing young residents to nearby cities, several have begun gaining population, driven largely by newcomers whose jobs no longer require them to be physically present in an urban office. Local commentators in some of these communities have adopted the informal term "zoom towns" to describe the phenomenon, a reference to the videoconferencing software many of these remote employees rely on to stay connected to employers based hundreds of kilometres away.

The appeal of such towns to remote workers is fairly straightforward: housing costs are typically far lower than in the metropolitan areas these newcomers previously lived in, commuting time is eliminated almost entirely, and many of the towns offer easy access to outdoor recreation such as hiking, skiing or lake activities that would have required a lengthy weekend drive from a city apartment. For the towns themselves, the arrival of relatively affluent remote workers can bring a welcome injection of spending into struggling local economies, supporting cafes, shops and services that might otherwise have closed as younger residents left for opportunities elsewhere.

The trend has not been universally welcomed, however. Long-time residents in several affected towns report that average house prices have risen sharply as newcomers, often earning salaries set for a far more expensive city, compete for a limited local housing stock, pricing out younger residents born in the area who work in lower-paying local jobs. Some towns have also struggled to expand infrastructure such as broadband capacity, water supply and schooling quickly enough to keep pace with a rapidly growing population, leading to strained local services in the short term.

Whether zoom towns represent a lasting demographic shift or a temporary phenomenon tied to a particular period of expanded remote-work flexibility remains uncertain. Some economists argue that as more employers begin requiring at least partial office attendance, the flow of newcomers into these towns may slow considerably, while others contend that the underlying appeal of lower housing costs and improved quality of life will continue to draw remote workers regardless of exactly how many days a week their employer expects them in a physical office.

Complete the sentences below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. Some rural towns have begun gaining population instead of losing young residents to nearby ______.
2. Local commentators use the informal term ______ to describe this phenomenon.
3. Housing costs in these towns are typically far lower than in the ______ areas newcomers previously lived in.
4. The arrival of remote workers can bring a welcome injection of ______ into struggling local economies.
5. Long-time residents report that average ______ have risen sharply as newcomers compete for limited housing.
6. Some towns have struggled to expand infrastructure such as broadband capacity, water supply and ______ quickly enough.
7. Some economists believe the flow of newcomers may slow as more employers require at least partial ______ attendance.