【正文】 GMAT Reading Comprehension Official GuideTable of ContentPassage 1 2Passage 2 4Passage 3 6Passage 4 9Passage 5 11Passage 6 13Passage 7 15Passage 8 17Passage 9 20Passage 10 22Passage 11 23Passage 12 25Passage 13 27Passage 14 28Passage 15 30Passage 16 32Passage 17 34Passage 18 36Passage 19 38Passage 20 39Passage 21 41Passage 22 43Passage 23 45Passage 24 46Passage 25 48Passage 26 49Passage 27 51Passage 28 53Passage 29 55Passage 30 57Passage 31 59Passage 32 62Passage 33 64Passage 34 66Passage 35 67Passage 36 70Passage 37 71Passage 38 72Passage 39 74Passage 40 76Passage 41 77Passage 42 78Passage 43 80Passage 44 81Passage 45 82Passage 46 83Passage 47 84Passage 48 86使用說明各題答案和解釋均隱藏在每篇文章后面，顯示的方法是鼠標(biāo)左鍵單擊“常用”工具欄“顯示/隱藏編輯標(biāo)記”按鈕。再次單擊又被隱藏。試題及答案均經(jīng)無數(shù)遍仔細(xì)校對(duì)，是官方答案，但錯(cuò)誤肯定在所難免。請(qǐng)各位不吝指正，不勝感謝！祝你成功！ READING COMPREHENSIONPassage 1Caffeine, the stimulant in coffee, has been called“the most widely used psychoactive substance on Earth .”Synder, Daly and Bruns have recently proposed thatcaffeine affects behavior by countering the activity in(5) the human brain of a naturally occurring chemical calledadenosine. Adenosine normally depresses neuron firingin many areas of the brain. It apparently does this byinhibiting the release of neurotransmitters, chemicalsthat carry nerve impulses from one neuron to the next.(10) Like many other agents that affect neuron firing,adenosine must first bind to specific receptors onneuronal membranes. There are at least two classesof these receptors, which have been designated A1 andA2. Snyder et al propose that caffeine, which is struc(15) turally similar to adenosine, is able to bind to both typesof receptors, which prevents adenosine from attachingthere and allows the neurons to fire more readily thanthey otherwise would.For many years, caffeine’s effects have been attri(20) buted to its inhibition of the production of phosphodiesterase, an enzyme that breaks down the chemicalcalled cyclic number of neurotransmitters exerttheir effects by first increasing cyclic AMP concentrations in target neurons. Therefore, prolonged periods at(25) the elevated concentrations, as might be brought aboutby a phosphodiesterase inhibitor, could lead to a greateramount of neuron firing and, consequently, to behavioral stimulation. But Snyder et al point out that thecaffeine concentrations needed to inhibit the production(30) of phosphodiesterase in the brain are much higher thanthose that produce stimulation. Moreover, other pounds that block phosphodiesterase’s activity are notstimulants.To buttress their case that caffeine acts instead by pre(35) venting adenosine binding, Snyder et al pared thestimulatory effects of a series of caffeine derivatives withtheir ability to dislodge adenosine from its receptors inthe brains of mice. “In general,” they reported, “theability of the pounds to pete at the receptors(40) correlates with their ability to stimulate lootion inthe mouse。 ., the higher their capacity to bind at thereceptors, the higher their ability to stimulate lootion.” Theophylline, a close structural relative of caffeineand the major stimulant in tea, was one of the most(45) effective pounds in both regards.There were some apparent exceptions to the generalcorrelation observed between adenosinereceptor bindingand stimulation. One of these was a pound called3isobuty11methylxanthine(IBMX), which bound very(50) well but actually depressed mouse lootion. Snyderet al suggest that this is not a major stumbling block totheir hypothesis. The problem is that the pound hasmixed effects in the brain, a not unusual occurrence withpsychoactive drugs. Even caffeine, which is generally(55) known only for its stimulatory effects, displays thisproperty, depressing mouse lootion at very lowconcentrations 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 theories and suggest a third theory that overes the problems encountered in the first two(D) describe an alternative hypothesis and provide evidence and arguments that support it（D）(E) challenge the validity of a theory by exposing the inconsistencies and contradictions in it2. According so 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(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 inhibits such release（D）(E) inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing3. In response to experimental results concerning IBMX, Snyder et al contended that it is not unmon for 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 lootion（A）(E) the ability to dislodge caffeine from receptors in the brain4. According to Snyder et al, all of the following pounds can bind to specific receptors in the brain EXCEPT(A) IBMX(B) caffeine(C) adenosine(D) theophylline（E）(E) phosphodiesterase5. Snyder et al suggest that caffeine’s ability to bind to A1 and A2 recept