1、国家公共英语四级（综合）练习试卷 3及答案与解析 Part B Directions: Read the following four texts. Answer the questions below each text by choosing A, B, C or D . Mark your answers on ANSWER SHEET 1. 0 We sometimes think humans are uniquely vulnerable to anxiety, but stress seems to affect the immune defenses of lower anim
2、als too. In one experiment, for example, behavioral immunologist Mark Laudenslager, at the University of Denver, gave mild electric shocks to 24 rats. Half the animals could switch off the current by turning a wheel in their enclosure, while the other half could not. The rats in the two groups were
3、paired so that each time one rat turned the wheel it protected both itself and its helpless partner from the shock. Laudenslager found that the immune response was depressed below normal in the helpless rats but not in those that could turn off the electricity. What he has demonstrated, he believes,
4、 is that lack of control over an event, not the experience itself, is what weakens the immune system. Other researchers agree. Jay Weiss, a psychologist at Duke University School of Medicine, has shown that animals who are allowed to control unpleasant stimuli don t develop sleep disturbances or cha
5、nges in brain chemistry typical of stressed rats. But if the animals are confronted with situations they have no control over, they later behave passively when faced with experiences they can control. Such findings reinforce psychologists suspicions that the experience or perception of helplessness
6、is one of the most harmful factors in depression. One of the most startling examples of how the mind can alter the immune response was discovered by chance. In 1975 psychologist Robert Ader at the University of Rochester School of Medicine conditioned mice to avoid saccharin by simultaneously feedin
7、g them the sweetener and injecting them with a drug that while suppressing their immune systems caused stomach upsets. Associating the saccharin with the stomach pains, the mice quickly learned to avoid the sweetener. In order to extinguish this dislike for the sweetener, Ader re-exposed the animals
8、 to saccharin, this time without the drug, and was astonished to find that those mice that had received the highest amounts of sweetener during their earlier conditioning died. He could only speculate that he had so successfully conditioned the rats that saccharin alone now served to weaken their im
9、mune systems enough to kill them. 1 Laudenslager s experiment showed that the immune system of those rats who could turn off the electricity _ . （ A） was strengthened （ B） was not affected （ C） was altered （ D） was weakened 2 According to the passage, the experience of helplessness causes rats to _
10、. （ A） try to control unpleasant stimuli （ B） turn off the electricity （ C） behave passively in controllable situations （ D） become abnormally suspicious 3 The reason why the mice in Ader s experiment avoided saccharin was that _ . （ A） they disliked its taste （ B） it affected their immune systems （
11、 C） it led to stomach pains （ D） they associated it with stomach 4 The passage tells us that the most probable reason for the death of the mice in Ader s experiment was that _ . （ A） they had been weakened psychologically by the saccharin （ B） the sweetener was poisonous to them （ C） their immune sy
12、stems had been altered by the mind （ D） they had taken too much sweetener during earlier conditioning 5 It can be concluded from the passage that the immune systems of animals _ . （ A） can be weakened by conditioning （ B） can be suppressed by drug injections （ C） can be affected by frequent doses of
13、 saccharin （ D） can be altered by electric shocks 5 Ever since Gregor Mendel s famous experiments with hybrid sweet peas, it has been known that there must be unitary elements within the cells which exert control over inherited characteristics, and for a long time there was considerable speculation
14、about what these were. These elements came to be known as genes, and although they were long treated as hypothetical constructs, a great deal of knowledge about them slowly accumulated. It came to be known, for example, that each gene had to be passed along virtually unchanged from generation to gen
15、eration; that there must be many thousands of these particles in every human cell, distributed unevenly among the twenty-three pairs of chromosomes; that each gene must occupy a very definite place (locus) on its chromosome; and that each pair of homologous chromosomes had to contain homologous asso
16、rtments of genes, arranged with few exceptions in precisely the same order on each member of the chromosome pail s. A wonderfully complex and fruitful system thus emerged about an aspect of the world which no one has ever directly observed. Let us now briefly turn to some of the newly acquired insig
17、hts which have greatly expanded the already impressive theory of genetics. Genes are, of course, too small to be seen even by the most powerful electron microscopes, but recent research by geneticists, microbiologists, and biochemists has rapidly advanced our information about their constitution and
18、 action. The chemical substance of which the genes and thus the chromosomes are made, is now known to be deoxyribonucleic acid (DNA), a giant molecule containing a double-spiral strand of material which embodies the genetic code. The chromosomes consist of long strands of DNA, which, although it is
19、capable of transmitting vastly complex “code messages“, is comprised of combinations of only four primary chemical subunits, or “code letters“. This great insight into the structure and functioning of genetic material, which was first proposed by James D. Watson and Francis H. C. Crick in 1953, invo
20、lves a new description of what genes are like. A gene is simply a specific portion of the double-spiral strand of DNA which consists of a particular combination of the code letters that spell out a particular code word. Various combinations of the four code letters, forming different code words, pro
21、vide the biochemical information used in the construction of the different proteins in the cell. Many of these proteins act as enzymes. The enzymes, as has been pointed out above, are the biological catalysts which direct all the chemical or metabolic reactions that are going on continuously in all
22、cells. These metabolic functions are, of course, the basis of all the physical growth and development of any living organism. The code is embodied in the DNA of the chromosomes and genes, but exactly how does this code deter mine the production of proteins. Obviously, the code must be transmitted to
23、 the sites at which the actual work of protein synthesis is carried out. The material which accomplishes this task is ribonucleic acid (RNA, a substance very similar to DNA and complementary to it. From the code site on the linear DNA molecule, which is the gene), RNA, the messenger, carries the cod
24、e to the cellular particles out into the cytoplasm of the cell, where proteins are manufactured. This messenger RNA provides the pattern, and another type of RNA, transfer RNA, collects from within the cytoplasm the raw materials, the amino acids, from which the proteins are made. With the pattern a
25、nd the materials, the proteins are formed, one step at a time. These proteins act as enzymes or biological catalysis. They exist in all living organisms and control their growth and function through the control of the chemical transformations involved in metabolism. A very large number of enzymes ar
26、e present in any living creature, and the absence or malformation of any enzyme can destroy the normal sequence of metabolism of a given biochemical substance. We can thus see that genetic activity takes the form of biochemical regulation, the genes determining the formation of enzymes. In this sens
27、e, all genetic disorders are primarily metabolic defects (Garrod, 1908). A defective or changed gene will in turn produce a change in the protein with which it is associated. The only result of such a change may be a slight alteration in the function of the protein, and there may thus be little or n
28、o observable effect. If the change or defect takes place within the code message for an essential element of the protein, however, the enzyme activity of this protein may be rendered completely inactive. If this happens, the result can be grave trouble: perhaps death, serious disease, or severe ment
29、al retardation due to poisoning of the central nervous system by a metabolite that is toxic to this system. The error in enzyme synthesis may begin to be important, so that the structure of the central nervous system is faulty almost from the beginning of embryonic life, or it may become important m
30、uch later in the life cycle. It is quite likely that, in the foreseeable future, many essential biochemical processes will be understood in terms of the precise genetic codes responsible for them. All of the amino acids have already yielded to such analysis; their codes have been identified. With un
31、derstanding may come control and prevention, such as may be possible by administration of the lacking enzymes, dietary control of substances which the individual is unable to metabolize, or transplantation of normal tissue to the diseased individual to correct the metabolic error. 6 Genes determine
32、through complex biochemical processes the structure and rate in which _ . （ A） proteins are formed （ B） chromosomes are paired （ C） enzymes are produced （ D） cells multiply 7 The genetic material in the nucleus of the cell is called the _ . （ A） cytoplasm （ B） chromosome （ C） chromatin （ D） gene 8 I
33、n man, the body cells contain how many pairs of chromosomes? （ A） Twenty-two. （ B） Twenty-three. （ C） Twenty-six. （ D） Forty-six. 9 From generation to generation genes _ . （ A） change location in the chromosome （ B） change order but not position in the chromosome （ C） are constantly changing their c
34、hemical makeup （ D） remain virtually unchanged 10 The genetic code is embodied in _ . （ A） the proteins of the genes （ B） the DNA （ C） a double-spiral atom （ D） genes 10 Many of the most damaging and life-threatening types of weather-torrential rains, severe thunderstorms, and tornadoes-begin quickl
35、y, strike suddenly, and dissipate rapidly, devastating small regions while leaving neighboring areas untouched. One such event, a tornado, struck the northeastern section of Edmonton, Alberta, in July 1987. Total damages from the tornado exceeded 250 million, the highest ever for any Canadian storm.
36、 Conventional computer models of the atmosphere have limited value in predicting short-lived local storms like the Edmonton tornado, because the available weather data are generally not detailed enough to allow computers to discern the subtle atmospheric changes that precede these storms. In most na
37、tions, for example, weather-balloon observations are taken just once every twelve hours at locations typically separated by hundreds of miles. With such limited data, conventional forecasting models do a much better job predicting general weather conditions over larger regions than they do forecasti
38、ng specific local events. Until recently, the observation-intensive approach needed for accurate, very short-range forecasts, or “Nowcasts“, was not feasible. The cost of equipping and operating many thousands of conventional weather stations was prohibitively high, and the difficulties involved in
39、rapidly collecting and processing the raw weather data from such a network were insurmountable. Fortunately, scientific and technological advances have overcome most of these problems. Radar systems, automated weather instruments, and satellites are all capable of making detailed, nearly continuous
40、observation over large regions at a relatively low cost. Communications satellites can transmit data around the world cheaply and instantaneously, and modern computers can quickly compile and analyzing this large volume of weather information. Meteorologists and computer scientists now work together
41、 to design computer programs and video equipment capable of transforming raw weather data into words, symbols, and vivid graphic displays that forecasters can interpret easily and quickly. As meteorologists have begun using these new technologies in weather forecasting offices, Nowcasting is becomin
42、g a reality. 11 What does the passage mainly discuss? （ A） Computers and weather. （ B） Dangerous storms. （ C） Weather forecasting. （ D） Satellites. 12 Why does the author state in the first paragraph that observations are taken “just once every twelve hours“? （ A） To indicate that the observations a
43、re timely. （ B） To show why the observations are of limited value. （ C） To compare data from balloons and computers. （ D） To give an example of international cooperation. 13 Which of the following is NOT mentioned as an advance in short-range weather forecasting? （ A） Weather balloons. （ B） Radar sy
44、stems. （ C） Automated instruments. （ D） Satellites. 14 With Nowcasting, it first became possible to provide information about _ . （ A） short-lived local storms （ B） radar networks （ C） long-range weather forecasts （ D） general weather conditions 15 Which of the following would best illustrate Nowcas
45、ting? （ A） A five-day forecast. （ B） A warning about a severe thunderstorm on the radio. （ C） The average rainfall for each month. （ D） A list of temperatures in major cities. 国家公共英语四级（综合）练习试卷 3答案与解析 Part B Directions: Read the following four texts. Answer the questions below each text by choosing A
46、, B, C or D . Mark your answers on ANSWER SHEET 1. 【知识模块】 Science and Technology 1 【听力原文】 见文章第一段倒数第二句。 1 【试题解析】 B 【知识模块】 Science and Technology 2 【听力原文】 见文章第二段倒数第二句。 2 【试题解析】 C 【知识模块】 Science and Technology 3 【听力原文】 见文章第三段第二句。 3 【试题解析】 D 【知识模块 】 Science and Technology 4 【听力原文】 结合文章第三段可以找出答案。 4 【试题解析
47、】 C 【知识模块】 Science and Technology 5 【听力原文】 见文章第三段最后一句。 5 【试题解析】 A 【知识模块】 Science and Technology 【知识模块】 Science and Technology 6 【听力原文】 见文章第一段中间。 6 【试题解析】 B 【知识模块】 Science and Technology 7 【听力原文】 见文章第一段开头。 7 【试题解析】 D 【知识模块】 Science and Technology 8 【听力原文】 见文章第一段第三句后半部分。 8 【试题解析】 B 【知识模块】 Science and
48、Technology 9 【听力原文】 见文章第一段第三句前半部分。 9 【试题解析】 D 【知识模块】 Science and Technology 10 【听力原文】 见文章第四段开头。 10 【试题解析】 B 【知识模块】 Science and Technology 【知识模块】 Science and Technology 11 【听力原文】 本篇的主题意义是气象预报。 11 【试题解析】 C 【知识模块】 Science and Technology 12 【听力原文】 通过这个例子，并通过与这个例子相关的以及后续的句子可以得出答案。 12 【试题解析】 B 【知识模块】 Science and Technology 13 【听力原文】 从第二段第三句可以排除其他三个选项。 13 【试题解析】 A 【知识模块】 Science and Technology 14 【听力原文】 见文章第一段。 14 【试题解析】 A 【知识模块】 Science and Technology 15 【听力原文】 从前几题和文章最后一句可以推断出此答案。 15 【试题解析】 B 【知识模块】 Science and Technology
