1、GMAT（ VERBAL）阅读模拟试卷 18及答案与解析 0 Caffeine, the stimulant in coffee, has been called “the most widely used psychoactive substance on Earth. “ Syn-der, Daly and Bruns have recently pro- posed that caffeine affects behavior by countering the activity in the human brain of a naturally occurring chemical c
2、alled adenosine. Adenosine normally depresses neuron firing in many areas of the brain. It apparently does this by inhibiting the release of neurotransmitters, chemicals that carry nerve impulses from one neuron to the next. Like many other agents that affect neuron firing, adenosine must first bind
3、 to specific receptors on neuronal membranes. There are at least two classes of these receptors, which have been designated Al and A2. Snyder et al propose that caffeine, which is structurally similar to adenosine, is able to bind to both types of receptors, which prevents adenosine from attaching t
4、here and allows the neurons to fire more readily than they oth- erwise would. For many years, caffeines effects have been attributed to its inhibition of the production of phosphodiesterase, an enzyme that breaks down the chemical called cyclic AMP. A number of neurotransmitters exert their effects
5、by first increasing cyclic AMP concentrations in target neurons. Therefore, prolonged periods at the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor, could lead to a greater amount of neuron firing and, consequently, to behavioral stimulation. But Snyder et al poi
6、nt out that the caf- feine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other compounds that block phosphodi- esterases activity are not stimulants. To buttress their case that caffeine acts instead b
7、y preventing adenosine binding, Snyder et al compared the stimulatory effects of a series of caffeine derivatives with their ability to dislodge adenosine from its receptors in the brains of mice. “In general,“ they reported, “the ability of the compounds to compete at the receptors correlates with
8、their ability to stimulate locomotion in the mouse; i. e., the higher their capacity to bind at the receptors, the higher their ability to stimulate locomotion. “ Theophylline, a close structural relative of caffeine and the major stimulant in tea, was one of the most effective compounds in both reg
9、ards. There were some apparent exceptions to the general correlation observed between adenosine-receptor binding and stimulation. One of these was a compound called 3-isobutyl-l-methylxan-thine(IBMX), which bound very well but actually depressed mouse locomo- tion. Snyder et al suggest that this is
10、not a major stumbling block to their hypothesis. The problem is that the compound has mixed effects in the brain, a not unusual occurrence with psychoac- tive drugs. Even caffeine, which is generally known only for its stimulatory effects, displays this property, depressing mouse locomotion at very
11、low concentrations and stimulating it at higher ones. 1 The primary purpose of the passage is to （ A） discuss a plan for investigation of a phenomenon that is not yet fully understood. （ B） present two explanations of a phenomenon and reconcile the differences between them. （ C） summarize two theori
12、es and suggest a third theory that overcomes the problems encountered in the first two. （ D） describe an alternative hypothesis and provide evidence and arguments that support it. （ E） challenge the validity of a theory by exposing the inconsistencies and contradictions in it. 2 Which of the followi
13、ng, if true, would most weaken the theory proposed by Snyder et al? （ A） At very low concentrations in the human brain, both caffeine and theophylline tend to have depressive rather than stimulatory effects on human behavior. （ B） The ability of caffeine derivatives at very low concentrations to dis
14、lodge adenosine from its receptors in mouse brains correlates well with their ability to stimulate mouse locomotion at these low concentrations. （ C） The concentration of cyclic AMP in target neurons in the human brain that leads to increased neuron firing can be produced by several different phosph
15、odiesterase inhibitors in addition to caffeine. （ D） The concentration of caffeine required to dislodge adenosine from its receptors in the human brain is much greater than the concentration that produces behavioral stimulation in humans. （ E） The concentration of IBMX required to dislodge adenosine
16、 from its receptors in mouse brains is much smaller than the concentration that stimulates locomotion in the mouse. 3 According to Snyder et al, caffeine differs from adenosine in that caffeine （ A） stimulates behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only.
17、 （ B） has mixed effects in the brain, whereas adenosine has only a stimulatory effect. （ C） increases cyclic AMP concentrations in target neurons, whereas adenosine decreases such concentrations. （ D） permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inh
18、ibits such release. （ E） inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing. 4 In response to experimental results concerning IBMX, Snyder et al contended that it is not uncommon for
19、 psychoactive drugs to have （ A） mixed effects in the brain. （ B） inhibitory effects on enzymes in the brain. （ C） close structural relationships with caffeine. （ D） depressive effects on mouse locomotion. （ E） the ability to dislodge caffeine from receptors in the brain. 5 The passage suggests that
20、 Snyder et al believe that if the older theory concerning caffeines effects were correct, which of the following would have to be the case? I . All neurotransmitters would increase the short-term concentration of cyclic AMP in target neurons. II . Substances other than caffeine that inhibit the prod
21、uction of phosphodiesterase would be stimulants. III . All concentration levels of caffeine that are high enough to produce stimulation would also inhibit the production of phosphodiesterase. （ A） I only （ B） I and II only （ C） I and III only （ D） II and III only （ E） I , II and III 6 According to S
22、nyder et al, all of the following compounds can bind to specific receptors in the brain EXCEPT （ A） IBMX. （ B） caffeine. （ C） adenosine. （ D） theophylline. （ E） phosphodiesterase. 7 Snyder et al suggest that caffeines ability to bind to Al and A2 receptors can be at least partially attributed to whi
23、ch of the following? （ A） The chemical relationship between caffeine and phosphodiesterase. （ B） The structural relationship between caffeine and adenosine. （ C） The structural similarity between caffeine and neurotransmitters. （ D） The ability of caffeine to stimulate behavior. （ E） The natural occ
24、urrence of caffeine and adenosine in the brain. 8 The author quotes Snyder et al in lines 52 - 59 most probably in order to （ A） reveal some of the assumptions underlying their theory. （ B） summarize a major finding of their experiments. （ C） point out that their experiments were limited to the mous
25、e. （ D） indicate that their experiments resulted only in general correlations. （ E） refute the objections made by supporters of the older theory. 9 The last paragraph of the passage performs which of the following functions? （ A） Describes a disconfirming experimental result and reports the explanat
26、ion given by Snyder et al in an attempt to reconcile this result with their theory. （ B） Specifies the basis for the correlation observed by Snyder et al and presents an explanation in an attempt to make the correlation consistent with the operation of psychoactive drugs other than caffeine. （ C） El
27、aborates the description of the correlation observed by Snyder et al and suggests an additional explanation in an attempt to make the correlation consistent with the older theory. （ D） Reports inconsistent experimental data and describes the method Snyder et al will use to reanalyze this data. （ E）
28、Provides an example of the hypothesis proposed by Snyder et al and relates this example to caffeines properties. 9 Archaeology as a profession faces two major problems. First, it is the poorest of the poor. Only paltry sums are available for excavating and even less is available for publishing the r
29、esults and preserving the sites once excavated. Yet archaeologists deal with priceless objects every day. Second, there is the problem of illegal excavation, resulting in muse- um-quality pieces being sold to the highest bidder. I would like to make an outrageous suggestion that would at one stroke
30、provide funds for archaeology and reduce the amount of illegal digging. I would propose that scientific archeological expeditions and governmental authorities sell excavated artifacts on the open market. Such sales would provide substan- tial funds for the excavation and preservation of archaeologic
31、al sites and the publication of results. At the same time, they would break the illegal excavators grip on the market, thereby de- creasing the inducement to engage in illegal activities. You might object that professionals excavate to acquire knowledge, not money. Moreover, ancient artifacts are pa
32、rt of our global cultural heritage, which should be available for all to appreciate, not sold to the highest bidder. I agree. Sell nothing that has unique artistic merit or scientific value. But, you might reply, everything that comes out of the ground has scientific value. Here we part company. The
33、oretically, you may be correct in claiming that every artifact has potential scientific val- ue. Practically, you are wrong. I refer to the thousands of pottery vessels and ancient lamps that are essentially duplicates of one another. In one small excavation in Cyprus, archaeolo- gists recently unco
34、vered 2,000 virtually indistinguishable small jugs in a single courtyard. Even precious royal seal impressions known as lmelekh handles have been found in abundance more than 4,000 examples so far. The basements of museums are simply not large enough to store the artifacts that are likely to be disc
35、overed in the future. There is not enough money even to catalogue the finds; as a result, they cannot be found again and become as inaccessible as if they had never been discovered. Indeed, with the help of a computer, sold artifacts could be more accessible than are the pieces stored in bulging mus
36、eum basements. Prior to sale, each could be photographed and the list of the purchasers could be maintained on the computer. A purchaser could even be required to agree to return the piece if it should become needed for scientific purposes. It would be unrealistic to suggest that illegal digging wou
37、ld stop if arti- facts were sold on the open market. But the demand for the clandestine product would be substantially reduced. Who would want an unmarked pot when another was available whose provenance was known, and that was dated strati-graphically by the professional archaeologist who excavated
38、it? 10 The primary purpose of the passage is to propose （ A） an alternative to museum display of artifacts. （ B） a way to curb illegal digging while benefiting the archaeological profession. （ C） a way to distinguish artifacts with scientific value from those that have no such value. （ D） the govern
39、mental regulation of archaeological sites. （ E） a new system for cataloguing duplicate artifacts. 11 The author implies that all of the following statements about duplicate artifacts are true EXCEPT: （ A） A market for such artifacts already exists. （ B） Such artifacts seldom have scientific value. （
40、 C） There is likely to be a continuing supply of such artifacts. （ D） Museums are well supplied with examples of such artifacts. （ E） Such artifacts frequently exceed in quality those already catalogued in museum collections. 12 Which of the following is mentioned in the passage as a disadvantage of
41、 storing artifacts in museum basements? （ A） Museum officials rarely allow scholars access to such artifacts. （ B） Space that could be better used for display is taken up for storage. （ C） Artifacts discovered in one excavation often become separated from each other. （ D） Such artifacts are often da
42、magded by variations in temperature and humidity. （ E） Such artifacts often remain uncata-logued and thus cannot be located once they are put in storage. 13 The author mentions the excavation in Cyprus(lines 43 - 50)to emphasize which of the following points? （ A） Ancient lamps and pottery vessels a
43、re less valuable, although more rare, than royal seal impressions. （ B） Artifacts that are very similar to each other present cataloguing difficulties to archaeologists. （ C） Artifacts that are not uniquely valuable, and therefore could be sold, are available in large quantities. （ D） Cyprus is the
44、most important location for unearthing large quantities of salable artifacts. （ E） Illegal sales of duplicate artifacts are widespread, particularly on the island of Cyprus. 14 The author s argument concerning the effect of the official sale of duplicate artifacts on illegal excavation is based on w
45、hich of the following assumptions? （ A） Prospective purchasers would prefer to buy authenticated artifacts. （ B） The price of illegally excavated artifacts would rise. （ C） Computers could be used to trace sold artifacts. （ D） Illegal excavators would be forced to sell only duplicate artifacts. （ E）
46、 Money gained from selling authenticated artifacts could be used to investigate and prosecute illegal excavators. 15 The author anticipates which of the following initial objections to the adoption of his proposal? （ A） Museum officials will become unwilling to store artifacts. （ B） An oversupply of
47、 salable artifacts will result and the demand for them will fall. （ C） Artifacts that would have been displayed in public places will be sold to private collectors. （ D） Illegal excavators will have an even larger supply of artifacts for resale. （ E） Counterfeiting of artifacts will become more comm
48、onplace. 16 The author implies that which of the following would occur if duplicate artifacts were sold on the open market? I . Illegal excavation would eventually cease completely. II. Cyprus would become the primary source of marketable duplicate artifacts. III. Archaeologists would be able to pub
49、lish the results of their excavations more frequently than they currently do. （ A） I only （ B） III only （ C） I and II only （ D） II and III only （ E） I , II and III 16 Federal efforts to aid minority businesses began in the 1960 s when the Small Business Administration(SBA)began making federally guaranteed loans and government-sponsored management and technical assistance available to minority business enterprises. While this program enabled many minority entrepreneurs to form new businesses, the results were disappointing, since managerial inexperience, unfavorable locations, a
