gre考试阅读真题解析(A)

　　The 1960s witnessed two profound social movements: the civil rights movement and the movement protesting the war in Vietnam. Although they overlapped in time, they were largely distinct. For a brief moment in 1967, however, it appeared that the two movements might unite under the leadership of Martin Luther King, Jr.

　　Kings role in the antiwar movement appears to require little explanation, since he was the foremost advocate of nonviolence of his time. But Kings stance on the Vietnam War cannot be explained in terms of pacifism alone. After all, he was something of a latecomer to the antiwar movement, even though by 1965 he was convinced that the role of the United States in the war was indefensible. Why then the two years that passed before he translated his private misgivings into public dissent? Perhaps he believed that he could not criticize American foreign policy without endangering the support for civil rights that he had won from the federal government.

　　17. According to the passage, the delay referred to in lines 12-15 is perhaps attributable to which of the following?

　　 Kings ambivalence concerning the role of the United States in the war in Vietnam

　　 Kings attempts to consolidate support for his leadership within the civil rights movement

　　 Kings desire to keep the leadership of the civil rights movement distinct from that of the antiwar movement

　　 Kings desire to draw support for the civil rights movement from the leadership of the antiwar movement

　　 Kings reluctance to jeopardize federal support for the civil rights movement

　　18. The author supports the claim that Kings stance on the Vietnam War cannot be explained in terms of pacifism alone by implying which of the following?

　　 There is little evidence that King was ever a student of pacifist doctrine.

　　 King, despite pacifist sympathies, was not convinced that the policy of the federal government in Vietnam was wrong.

　　 Kings belief in nonviolence was formulated in terms of domestic policy rather than in terms of international issues.

　　 Had Kings actions been based on pacifism alone, he would have joined the antiwar movement earlier than he actually did.

　　 Opponents of United States foreign policy within the federal government convinced King of their need for support.

　　19. Which of the following can be inferred from the passage about the movement opposing the war in Vietnam?

　　 It preceded the civil rights movement.

　　 It began in 1965.

　　 It was supported by many who otherwise opposed public dissent.

　　 It drew support from most civil rights leaders.

　　 It was well underway by 1967.

　　20. Which of the following best describes the passage?

　　 It discusses an apparent inconsistency and suggests a reason for it.

　　 It outlines a sequence of historical events.

　　 It shows why a commonly held view is inaccurate.

　　 It evaluates an explanation and finally accepts that explanation.

　　 It contrasts two views of an issue.

　　What causes a helix in nature to appear with either a dextral twist or a sinistral twist is one of the most intriguing puzzles in the science of form. Most spiral-shaped snail species are predominantly dextral. But at one time , handedness was equally distributed within some snail species that have become predominantly dextral or, in a few species, predominantly sinistral. What mechanisms, control handedness and keep left-handedness rare?

　　It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable. But left- and right-handed snails are not actually true mirror images of one another. Their shapes are noticeably different. Sinistral rarity might, then, be a consequence of possible disadvantages conferred by these other concomitant structural features. In addition, perhaps left- and right-handed snails cannot mate with each other, having incompatible twist directions. Presumably an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right-and left-handed populations.

　　But this evolutionary mechanism combining dissymmetry, anatomy, and chance does not provide an adequate explanation of why right-handedness should have become predominant. It does not explain, for example, why the infrequent unions between snails of opposing hands produce fewer offspring of the rarer than the commoner form in species where each parent contributes equally to handedness. Nor does it explain why, in a species where one parent determines handedness, a brood is not exclusively right- or left-handed when the offspring would have the same genetic predisposition. In the European pond snail Lymnaea peregra, a predominantly dextral species whose handedness is maternally determined, a brood might be expected to be exclusively right or left-handedand this often occurs. However, some broods possess a few snails of the opposing hand, and in predominantly sinistral broods, the incidence of dextrality is surprisingly high.

　　Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right or left-handedness. In the case of Lymnaea peregra, studies indicate that a dextral gene is expressed during egg formation; i.e., before egg fertilization, the gene produces a protein, found in the cytoplasm of the egg, that controls the pattern of cell division and thus handedness. In experiments, an injection of cytoplasm from dextral eggs changes the pattern of sinistral eggs, but an injection from sinistral eggs does not influence dextral eggs. One explanation for the differing effects is that all Lymnaea peregra eggs begin left-handed but most switch to being right-handed. Thus, the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself.

　　21. Which of the following would serve as an example of concomitant structural features that might disadvantage a snail of the rarer form?

　　 A shell and body that are an exact mirror image of a snail of the commoner form

　　 A smaller population of the snails of the rarer form

　　 A chip or fracture in the shell caused by an object falling on it

　　 A pattern on the shell that better camouflages it

　　 A smaller shell opening that restricts mobility and ingestion relative to that of a snail of the commoner form

　　22. The second paragraph of the passage is primarily concerned with offering possible reasons why

　　 it is unlikely that evolutionary mechanisms could discriminate against sinistral snails

　　 sinistrality is relatively uncommon among snail species

　　 dextral and sinistral populations of a snail species tend to intermingle

　　 a theory based on a developmental mechanism inadequately accounts for the predominance of dextrality across snail species

　　 dextral snails breed more readily than sinistral snails, even within predominantly sinistral populations

　　23. In describing the evolutionary mechanism , the author mentions which of the following?

　　 The favorable conditions for nurturing new offspring

　　 The variable environmental conditions that affect survival of adult snails

　　 The availability of potential mates for breeding

　　 The structural identity of offspring to parents of the same hand

　　 The frequency of unions between snails of different species

　　24. According to the passage, which of the following is true of Lymnaea peregra?

　　 Handedness within the species was at one time equally distributed between left and right.

　　 Under laboratory conditions, dextral eggs from Lymnaea peregra can be artificially induced to develop into sinistral snails.

　　 Broods of Lymnaea peregra are, without variation, exclusively sinistral or dextral.

　　 Handedness in Lymnaea peregra offspring is determined by only one of the parents.

　　 Geographic factors have played a larger role than has genetics in the evolution of the species.

　　25. The passage implies that in Lymnaea peregra, there will generally be

　　 more offspring of the nondominant hand in broods where handedness is determined after, rather than before, fertilization

　　 a sinistral gene that produces a protein in the cytoplasm of the egg cell

　　 fewer sinistral offspring in dextral broods than dextral offspring in sinistral broods

　　 equal numbers of exclusively left-and right-handed broods

　　 an increasing occurrence of left-handedness in successive broods

　　26. It can be inferred from the passage that a predominantly sinistral snail species might stay predominantly sinistral for each of the following reasons EXCEPT for

　　 a developmental mechanism that affects the cell-division pattern of snails

　　 structural features that advantage dextral snails of the species

　　 a relatively small number of snails of the same hand for dextral snails of the species to mate with

　　 anatomical incompatibility that prevents mating between snails of opposing hands within the species

　　 geographic separation of sinistral and dextral populations

　　27. Which of the following accurately describes the relationship between the evolutionary and developmental theories discussed in the passage?

　　 Although the two theories reach the same conclusion, each is based on different assumptions.

　　 They present contradictory explanations of the same phenomenon.

　　 The second theory accounts for certain phenomena that the first cannot explain.

　　 The second theory demonstrates why the first is valid only for very unusual, special cases.

　　 They are identical and interchangeable in that the second theory merely restates the first in less technical terms.

答案：17-27：EDEAEBCDCBC