Trắc nghiệm Tiếng anh 12 Đọc hiểu có đáp án (Phần 6)

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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. 

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In the past, both men and women were expected to be married at quite young ages. Marriages were generally arranged by parents and family, with their children having little chance to say no in the matter. In the past it was not surprising to find that a bride and groom had only just met on the day of their engagement or marriage. In modern Vietnam, this has changed completely as people choose their own marriage-partners based on love, and in consideration primarily to their own needs and wants. Moreover early marriage is quite illegal. The traditional Vietnamese wedding is one of the most important of traditional Vietnamese occasions. Regardless of westernization, many of the age-old customs practiced in a traditional Vietnamese wedding continue to be celebrated by both Vietnamese in Vietnam and overseas, often combining both western and eastern elements. Besides the wedding ceremony, there is also an engagement ceremony which takes place usually half a year or so before the wedding. Due to the spiritual nature of the occasion, the date and time of the marriage ceremony are decided in advance by a fortune teller. The traditional Vietnamese wedding consists of an extensive array of ceremonies: the first is the ceremony to ask permission to receive the bride, the second is the procession to receive the bride (along with the ancestor ceremony at her house), the third is to bring the bride to the groom's house for another ancestor ceremony and to welcome her into the family, then the last is a wedding banquet. The number of guests in attendance at these banquets is huge, usually in the hundreds. Several special dishes are served. Guests are expected to bring gifts, often money, which the groom and bride at one point in the banquet will go from table to table collecting.

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After two decades of growing student enrollments and economic prosperity, business schools in the United States have started to face harder times. Only Harvard's MBA School has shown a substantial increase in enrollment in recent years. Both Princeton and Stanford have seen decreases in their enrollments. Since 1990, the number of people receiving Masters in Business Administration (MBA) degrees, has dropped about 3 percent to 75,000, and the trend of lower enrollment rates is expected to continue. There are two factors causing this decrease in students seeking an MBA degree. The first one is that many graduates of four-year colleges are finding that an MBA degree does not guarantee a plush job on Wall Street, or in other financial districts of major American cities. Many of the entry-level management jobs are going to students graduating with Master of Arts degrees in English and the humanities as well as those holding MBA degrees. Students have asked the question, "Is an MBA degree really what I need to be best prepared for getting a good job?" The second major factor has been the cutting of American payrolls and the lower number of entry-level jobs being offered. Business needs are changing, and MBA schools are struggling to meet the new demands. 

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It is common knowledge that the Internet, often called simply Net, is the largest network of computers in the world. It connects businesses, institutions and individuals. One of its main uses is communication, and electronic mail, or e-mail, is probably the most popular use of the Internet. Although the Net is widely used by people of all ages, many adults are now concerned about is possibly harmful effects on young people.  example, chatting is a popular activity on the Net. Electronic chat rooms are Internet sites where people can communicate directly with one another. These chat rooms are quite popular with children. However, adults with inappropriate or illegal motives may also join chat rooms. Sometimes these adults try to arrange face-to-face meetings with unsuspecting victims. There have cases where children have been tricked by adults who pass them off as children and who give information that provokes to be entirely false. Another cause of worry for parents is the huge amount of information accessible to users of the Internet. Not all information on the Net is accurate, and some deliberately misleading. In addition, parents worry that children may be exposed to harmful material. For instance, there are innumerable sites where explicitly violent or pornographic scenes are shown. Moreover, many organizations maintain hate sites that present racist or politically extreme views. The Internet has tremendous potential for good, but it has also led to some serious questions about appropriate use. Where protection of their children is needed, parents should not hesitate to intervene. Unlimited freedom on the Net can lead to serious, sometimes dangerous, consequences. 

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Plants are subject to attack and infection by a remarkable variety of symbiotic species and have evolved a diverse array of mechanisms designed to frustrate the potential colonists. These can be divided into preformed or passive defense mechanisms and inducible or active systems. Passive plant defense comprises physical and chemical barriers that prevent entry of pathogens, such as bacteria, or render tissues unpalatable or toxic to the invader. The external surfaces of plants, in addition to being covered by an epidermis and a waxy cuticle, often carry spiky hairs known as trichomes, which either prevent feeding by insects or may even puncture and kill insect larvae. Other trichomes are sticky and glandular and effectively trap and immobilize insects. If the physical barriers of the plant are breached, then preformed chemicals may inhibit or kill the intruder, and plant tissues contain a diverse array of toxic or potentially toxic substances, such as resins, tannins, glycosides, and alkaloids, many of which are highly effective deterrents to insects that feed on plants. The success of the Colorado beetle in infesting potatoes, for example, seems to be correlated with its high tolerance to alkaloids that normally repel potential pests. Other possible chemical defenses, while not directly toxic to the parasite, may inhibit some essential step in the establishment of a parasitic relationship. For example, glycoproteins in plant cell walls may inactivate enzymes that degrade cell walls. These enzymes are often produced by bacteria and fungi. Active plant defense mechanisms are comparable to the immune system of vertebrate animals, although the cellular and molecular bases are fundamentally different. Both, however, are triggered in reaction to intrusion, implying that the host has some means of recognizing the presence of a foreign organism. The most dramatic example of an inducible plant defense reaction is the hypersensitive response. In the hypersensitive response, cells undergo rapid necrosis ― that is, they become diseased and die ― after being penetrated by a parasite ; the parasite itself subsequently ceases to grow and is therefore restricted to one or a few cells around the entry site. Several theories have been put forward to explain the bases of hypersensitive resistance. 

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Many ants forage across the countryside in large numbers and undertake mass migrations; these activities proceed because one ant lays a trail on the ground for the others to follow. As a worker ant returns home after finding a source of food, it marks the route by intermittently touching its stinger to the ground and depositing a tiny amount of trail pheromone – a mixture of chemicals that delivers diverse messages as the context changes. These trails incorporate no directional information and may be followed by other ants in either direction. Unlike some other messages, such as the one arising from a dead ant, a food trail has to be kept secret from members of other species. It is not surprising then that ant species use a wide variety of compounds as trail pheromones. Ants can be extremely sensitive to these signals. Investigators working with the trail pheromone of the leafcutter ant Atta texana calculated that one milligram of this substance would suffice to lead a column of ants three times around Earth. The vapor of the evaporating pheromone over the trail guides an ant along the way, and the ant detects this signal with receptors in its antennae. A trail pheromone will evaporate to furnish the highest concentration of vapor right over the trail, in what is called a vapor space. In following the trail, the ant moves to the right and left, oscillating from side to side across the line of the trail itself, bringing first one and then the other antenna into the vapor space. As the ant moves to the right, its left antenna arrives in the vapor space. The signal it receives causes it to swing to the left, and the ant then pursues this new course until its right antenna reaches the vapor space. It then swings back to the right, and so weaves back and forth down the trail. 

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On the fourth Thursday in November, in houses around the United States, families get together for a feast, or a large meal. Almost all families eat turkey and cranberry sauce for this meal, and have pumpkin pie for dessert. This feast is part of a very special day, the holiday of Thanksgiving. In 1620 the Pilgrims made a difficult trip across the ocean from England. They landed in what is now Massachusetts. In England the Pilgrims had not been allowed to freely practice their religion. So they went to the New World in search of religious freedom. The Pilgrims' first winter was very hard. Almost half the group died of cold, hunger and disease. But the Indians of Massachusetts taught the Pilgrims to plant corn, to hunt and to fish. When the next fall came, the Pilgrims had plenty of food. They were thankful to God and the Indians and had a feast to give thanks. They invited the Indians to join them. This was the first Thanksgiving. Thanksgiving became a national holiday many years later because of the effort of a woman named Sarah Hale. For forty years Sarah Hale wrote to each president and asked for a holiday of Thanksgiving. At last she was successful. In 1863 President Lincoln declared Thanksgiving a holiday. How much is Thanksgiving today like the Pilgrims’ Thanksgiving? In many ways they are different. For example, historians think that the Pilgrims ate deer, not turkey. The idea of Thanksgiving, though, is very much the same: Thanksgiving is a day on which we celebrate and give thanks.