Gre阅读真题200篇 brief contents
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One explanation for the tendencyOne explanation for the tendency of animals to be more vigilant in smaller groups than in larger ones assumes that the vigilant behavior—looking up, for example—is aimed at predators. If individuals on the edge of a group are more vigilant because they are at greater risk of being captured, then individuals on average would have to be more vigilant in smaller groups, because the animals on the periphery of a group form a greater proportion of the whole group as the size of the group diminishes. However, a different explanation is necessary in cases where the vigilant behavior is not directed at predators. J. Krebs has discovered that great blue herons look up more often when in smaller flocks than when in larger ones, solely as a consequence of poor feeding conditions. Krebs hypothesizes that the herons in smaller flocks are watching for herons that they might follow to better feeding pools, which usually attract larger numbers of the birds. 17. It can be inferred from the passage that in species in which vigilant behavior is directed at predators, the tendency of the animals to be more vigilant in smaller groups than in larger ones would most likely be minimized if which of the following were true? (A) The vigilance of animals on the periphery of a group always exceeded that of animals located in its interior, even when predators were not in the area. (B) The risk of capture for individuals in a group was the same, whether they were located in the interior of the group or on its periphery. (C) Animals on the periphery of a group tended to be less capable of defending themselves from attack by predators than animals located in the interior of the group. (D) Animals on the periphery of a group tended to bear marks that were more distinctive to predators than animals located in the interior of the group.（B） (E) Animals on the periphery of a group tended to have shorter life spans than animals located in the interior of the group. 18. Which of the following best describes the relationship of the second paragraph to the first? (A) The second paragraph relies on different evidence in drawing a conclusion similar to that expressed in the first paragraph. (B) The second paragraph provides further elaboration on why an assertion made at the end of the first paragraph proves to be true in most cases. (C) The second paragraph provides additional information in support of a hypothesis stated in the first paragraph. (D) The second paragraph provides an example of a case in which the assumption described in the first paragraph is unwarranted.（D） (E) The second paragraph describes a phenomenon that has the same cause as the phenomenon described in the first paragraph. 19. It can be inferred from the passage that the author of the passage would be most likely to agree with which of the following assertions about vigilant behavior? (A) The larger the group of animals, the higher the probability that individuals in the interior of the group will exhibit vigilant behavior. (B) Vigilant behavior exhibited by individuals in small groups is more effective at warding off predators than the same behavior exhibited by individuals in larger groups. (C) Vigilant behavior is easier to analyze in species that are preyed upon by many different predators than in species that are preyed upon by relatively few of them. (D) The term “vigilant,” when used in reference to the behavior of animals, does not refer exclusively to behavior aimed at avoiding predators.（D） (E) The term “vigilant,” when used in reference to the behavior of animals, usually refers to behavior exhibited by large groups of animals. 20. The passage provides information in support of which of the following assertions? (A) The avoidance of predators is more important to an animal’s survival than is the quest for food. (B) Vigilant behavior aimed at predators is seldom more beneficial to groups of animals than to individual animals. (C) Different species of animals often develop different strategies for dealing with predators. (D) The size of a group of animals does not necessarily reflect its success in finding food.（E） (E) Similar behavior in different species of animals does not necessarily serve the same purpose.
Experiments show that insects can function as pollinators of cycads (cycad: n.苏铁类), rare, palmlike tropical plants. Furthermore, cycads removed from their native habitats—and therefore from insects native to those habitats—are usually infertile. Nevertheless, anecdotal reports of wind pollination in cycads cannot be ignored. The structure of cycads male cones is quite consistent with the wind dispersal of pollen, clouds of which are released from some of the larger cones. The male cone of Cycas circinalis, for example, sheds almost 100 cubic centimeters of pollen, most of which is probably dispersed by wind. Still, many male cycad cones (a mass of ovule-bearing or pollen-bearing scales or bracts in trees of the pine family or in cycads that are arranged usually on a somewhat elongated axis (松树的)球果) are comparatively small and thus produce far less pollen. Furthermore, the structure of most female cycad cones seems inconsistent with direct pollination by wind. Only in the Cycas (cycas: n.[植]铁树,苏铁,凤尾松) genus are the females’ ovules (ovule: n.胚珠, 卵子) accessible to airborne pollen, since only in this genus are the ovules surrounded by a loose aggregation of megasporophylls (megasporophyll: [植]大孢子叶) rather than by a tight cone. 24. According to the passage, the size of a male cycad cone directly influences which of the following? (A) The arrangement of the male cone’s structural elements (B) The mechanism by which pollen is released from the male cone (C) The degree to which the ovules of female cycads are accessible to airborne pollen (D) The male cone’s attractiveness to potential insect pollinators（E） (E) The amount of pollen produced by the male cone 25. The passage suggests that which of the following is true of the structure of cycad cones? (A) The structure of cycad cones provides conclusive evidence in favor of one particular explanation of cycad pollination. (B) The structure of cycad cones provides evidence concerning what triggers the first step in the pollination process. (C) An irresolvable discrepancy exists between what the structure of most male cycad cones suggests about cycad pollination and what the structure of most female cones suggests about that process. (D) The structure of male cycad cones rules out a possible mechanism for cycad pollination that is suggested by the structure of most female cycad cones.（E） (E) The structure of male cycad cones is consistent with a certain means of cycad pollination, but that means is inconsistent with the structure of most female cycad cones. 26. The evidence in favor of insect pollination of cycads presented in lines 2-4 would be more convincing if which of the following were also true? (A) Only a small variety of cycad species can be successfully transplanted. (B) Cycads can sometimes be pollinated by means other than wind or insects. (C) Insects indigenous to regions to which cycads are transplanted sometimes feed on cycads. (D) Winds in the areas to which cycads are usually transplanted are similar to winds in cycads’ native habitats.（D） (E) The transplantation of cycads from one region to another usually involves the accidental removal and introduction of insects as well. 27. The passage suggests that which of the following is true of scientific investigations of cycad pollination? (A) They have not yet produced any systematic evidence of wind pollination in cycads. (B) They have so far confirmed anecdotal reports concerning the wind pollination of cycads. (C) They have, until recently, produced little evidence in favor of insect pollination in cycads. (D) They have primarily been carried out using cycads transplanted from their native habitats.（A） (E) They have usually concentrated on describing the physical characteristics of the cycad reproductive system.
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