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The cause of tooth decay is acid, which is produced by bacteria in the mouth. The acid removes minerals from tooth enamel, allowing tooth decay to begin; the saliva in your mouth encourages remineralization and neutralizes the acid. The rate at which bacteria in the mouth produce acid depends on the amount of plaque on the teeth, the composition of the microbial flora, and whether the bacteria of the plaque have been "primed" by frequent exposure to sugar. To keep your teeth healthy, a regular dental hygiene program should be followed.

Removing plaque with a toothbrush and dental floss temporarily reduces the numbers of bacteria in the mouth and thus reduces tooth decay. It also makes the surfaces of the teeth more accessible, enabling saliva to neutralize acid and remineralize lesions. If fluoride is present in drinking water when teeth are forming, some fluoride is incorporated into the enamel of the teeth, making them more resistant to attack by acid. Fluoride toothpaste seems to act in another way, by promoting the remineralization of early carious lesions.

In addition to a regular dental hygiene program, a good way to keep your teeth healthy is to reduce your intake of sweet food. The least cavity-causing way to eat sweets is to have them with meals and not between. The number of times you eat sweets rather than the total amount determines how much harmful acid the bacteria in your saliva produce. But the amount of sweets influences the quality of your saliva. Avoid, if you can, sticky sweets that stay in your mouth a long time. Also try to brush and floss your teeth after eating sugary foods. Even rinsing your mouth with water is effective.

Whenever possible, eat foods with fiber, such as raw carrot sticks, apples, celery sticks, etc., that scrape off plaque, acting as a toothbrush. Cavities can be greatly reduced if these rules are followed when eating sweets.

What does this passage mainly discuss?

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One of the most important social developments that helped to make possible a shift in thinking about the role of public education was the effect of the baby boom of the 1950's and 1960's on the schools. In the 1920's, but especially in the Depression conditions of the 1930's, the United States experienced a declining birth rate – every thousand women aged fifteen to forty-four gave birth to about 118 live children in 1920, 89.2 in 1930, 75.8 in 1936, and 80 in 1940. With the growing prosperity brought on by the Second WorldWar and the economic boom that followed it, young people married and established households earlier and began to raise larger families than had their predecessors during the Depression. Birth rates rose to 102 per thousand in 1946, 106.2 in 1950, and 118 in 1955. Although economics was probably the most important determinant, it is not the only explanation for the baby boom. The increased value placed on the idea of the family also helps to explain this rise in birth rates. The baby boomers began streaming into the first grade by the mid-1940's and became a flood by 1950. The public school system suddenly found itself “overtaxed”. While the number of schoolchildren rose because of wartime and postwar conditions, these same conditions made the schools even less prepared to cope with the flood. The wartime economy meant that few new schools were built between 1940 and 1945. Moreover, during the war and in the boom times that followed large numbers of teachers left their profession for better-paying jobs elsewhere in the economy.

Therefore, in the 1950's and 1960's, the baby boom hit an antiquated and inadequate school system. Consequently, the "custodial rhetoric" of the 1930's and early 1940's no longer made sense; that is, keeping youths aged sixteen and older out of the labor market by keeping them in school could no longer be a high priority for an institution unable to find space and staff to teach younger children aged five to sixteen. With the baby boom, the focus of educators and of laymen interested in education inevitably turned toward the lower grades and back to basic academic skills and discipline. The system no longer had much interest in offering nontraditional, new, and extra services to older youths.

Question 47: What does the passage mainly discuss?

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According to anthropologists, people in pre-industrial societies spent 3 to 4 hours per day or about 20 hours per week doing the work necessary for life. Modern comparison of the amount of work performed per week, however, began with the Industrial Revolution (1760-1840) when 10 to 12-hour workdays with six workdays per week were the norm. Even with extensive time devoted to work, however, both incomes and standards of living were low. As incomes rose near the end of the Industrial Revolution, it became increasingly common to treat Saturday afternoons as a half-day holiday. The half- day holiday had become standard practice in Britain by the 1870s, but did not become common in the United States until the 1920s.

In the United States, the first third of the twentieth century saw the workweek move from 60 hours per week to just under 50 hours by the start of the 1930s. In 1914 Henry Ford reduced daily work hours at his automobile plants from 9 to 8. In 1926 he announced that henceforth his factories would close for the entire day on Saturday. At the time, Ford received criticism from other firms such as United States Steel and Westinghouse, but the idea was popular with workers.

The Depression years of the 1930s brought with them the notion of job sharing to spread available work around; the workweek dropped to a modem low for the United States of 35 hours. In 1938 the Fair Labor Standards Act mandated a weekly maximum of 40 hours to begin in 1940, and since that time the 8-hour day, 5-day workweek has been the standard in the United States. Adjustments in various places, however, show that this standard is not immutable. In 1987, for example, German metalworkers struck for and received a 37.5-hour workweek; and in 1990 many workers in Britain won a 37-hour week. Since 1989, the Japanese government has moved from a 6 to a 5-day workweek and has set a national target of 1,800 work hours per year for the average worker. The average amount of work per year in Japan in 1989 was 2,088 hours per worker, compared to 1,957 for the United States and 1,646 for France.

Question 40: What does the passage mainly discuss?

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Since water is the basis of life, composing the greater part of the tissues of all living things, the crucial problem of desert animals is to survive in a world where sources of flowing water are rare. And since man’s inexorable necessity is to absorb large quantities of water at frequent intervals, he can scarcely comprehend that many creatures of the desert pass their entire lives without a single drop.

Uncompromising as it is, the desert has not eliminated life but only those forms unable to withstand its desiccating effects. No moist- skinned, water-loving animals can exist there. Few large animals are found. The giants of the North American desert are the deer, the coyote, and the bobcat. Since desert country is open, it holds more swift-footed running and leaping creatures than the tangled forest. Its population is largely nocturnal, silent, filled with reticence, and ruled by stealth. Yet they are not emaciated.

Having adapted to their austere environment, they are as healthy as animals anywhere else in the word. The secret of their adjustment lies in the combination of behavior and physiology. None could survive if, like mad dogs and Englishmen, they went out in the midday sun; many would die in a matter of minutes. So most of them pass the burning hours asleep in cool, humid burrows underneath the ground, emerging to hunt only by night. The surface of the sun-baked desert averages around 150 degrees, but 18 inches down the temperature is only 60 degrees.

Question 32: The author mentions all the following as examples of the behavior of desert animals EXCEPT

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There are a number of natural disasters that can strike across the globe. Two that are frequently linked to one another are earthquakes and tsunamis. Both of them can cause a great amount of devastation when they hit. However, tsunamis are the direct result of earthquakes and cannot happen without them.

The Earth has three main parts. They are the crust, the mantle, and the core. The crust is the outer layer of the Earth. It is not a single piece of land. Instead, it is comprised of a number of plates. There are a few enormous plates and many smaller ones. These plates essentially rest upon the mantle, which is fluid. As a result, the plates are in constant - yet slow - motion. The plates may move away from or toward other plates. In some cases, they collide violently with the plates adjoining them. The movement of the plates causes tension in the rock. Over a long time, this tension may build up. When it is released, an earthquake happens.

Tens of thousands of earthquakes happen every year. The vast majority are so small that only scientific instruments can perceive them. Others are powerful enough that people can feel them, yet they cause little harm or damage. More powerful earthquakes, however, can cause buildings, bridges, and other structures to collapse. They may additionally injure and kill thousands of people and might even cause the land to change its appearance.

Since most of the Earth’s surface is water, numerous earthquakes happen beneath the planet’s oceans. Underwater earthquakes can cause the seafloor to move. This results in the displacement of water in the ocean . When this occurs , a tsunami may form. This is a wave that forms on the surface and moves in all directions from the place where the earthquake happened. A tsunami moves extremely quickly and can travel thousands of kilometres. As it approaches land, the water near the coast gets sucked out to sea. This causes the tsunami to increase in height. Minutes later, the tsunami arrives. A large tsunami - one more than ten meters in height- can travel far inland. As it does that, it can flood the land, destroy human settlements, and kill large numbers of people.

Which of the following is true regarding the crust?

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Fish that live on the sea bottom benefit by being flat and hugging the contours. There are two very different types of flatfish and they have evolved in very separate ways. The skates and rays, relatives of the sharks have become flat in what might be called the obvious way. Their bodies have grown out sideways to form great “wings” They look as though they have been flattened but have remained symmetrical and “the right way up”. Conversely fish such as plaice, sole, and halibut have become flat in a different way. There are bony fish which have a marked tendency to be flattened in a vertical direction; they are much “taller” than they are wide. They use their whole vertically flattened bodies as swimming surfaces, which undulate through the water as they move. Therefore when their ancestors migrated to the seabed, they lay on one side than on their bellies. However, this raises the problem that one eye was always looking down into the sand and was effectively useless - In evolution this problem was solved by the lower eye “moving” around the other side. We see this process of moving around enacted in the development of every young bony flatfish. It starts life swimming near the surface, and is symmetrical and vertically flattened, but then the skull starts to grow in a strange asymmetrical twisted fashion, so that one eye for instance the left, moves over the top of the head upwards, an old Picasso - like vision. Incidentally, some species of 20 flatfish settle on the right side, others on the left, and others on either side.

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