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GRE阅读模拟在线练习附答案一百六十七

2018-11-26 16:44:00来源：网络

　　对于GRE阅读部分，最好的练习资料就是模拟资料。通过模拟练习，可以让大家更好地发现自己在备考中的不足，重点攻克自己的短板内容，有效的提升阅读分数。下面新东方在线GRE频道为大家整理了详细的内容，供大家参考。

点击查看》》》【GRE阅读真题在线练习附答案汇总】

　　Many theories have been formulated to explain the role of grazers such as zooplankton in controlling the amount of planktonic algae (phytoplankton) in lakes.The first theories of such grazer control were merely based on observations of negative correlations between algal and zooplankton numbers. A low number of algal cells in the presence of a high number of grazers suggested,but did not prove,that the grazers had removed most of the algae. The converse observation, of the absence of grazers in areas of high phytoplankton concentration, led Hardy to propose his principle of animal exclusion, which hypothesized that phytoplankton produced a repellent that excluded grazers from regions of high phytoplankton concentration. This was the first suggestion of algal defenses against grazing.

　　Perhaps the fact that many of these first studies considered only algae of a size that could be collected in a net (net phytoplankton), a practice that overlooked the smaller phytoplankton (nannoplankton) that we now know grazers are most likely to feed on, led to a de-emphasis of the role of grazers in subsequent research. Increasingly, as in the individual studies of Lund, Round, and Reynolds, researchers began to stress the importance of environmental factors such as temperature, light, and water movements in controlling algal numbers. These environmental factors were amenable to field monitoring and to simulation in the laboratory. Grazing was believed to have some effect on algal numbers, especially after phytoplankton growth rates declined at the end of bloom periods, but grazing was considered a minor component of models that predicted algal population dynamics.

　　The potential magnitude of grazing pressure on freshwater phytoplankton has only recently been determined empirically. Studies by Hargrave and Geen estimated natural community graz-ing rates by measuring feeding rates of individual zooplankton species in the laboratory and then computing community grazing rates for field conditions using the known population density of grazers. The high estimates of grazing pressure postulated by these researchers were not fully accepted, however, until the grazing rates of zooplankton were determined directly in the field, by means of new experimental techniques. Using a specially prepared feeding chamber, Haney was able to record zooplankton grazing rates in natural field conditions. In the periods of peak zooplankton abundance, that is, in the late spring and in the summer, Haney recorded maximum daily community grazing rates, for nutrient-poor lakes and bog lakes, respectively, of 6.6 percent and 114 percent of daily phytoplankton production. Cladocerans had higher grazing rates than copepods, usually accounting for 80 percent of the community grazing rate. These rates varied seasonally, reaching the lowest point in the winter and early spring.Haney's thorough research provides convincing field evidence that grazers can exert significant pressure on phytoplankton population.

　　The author most likely mentions Hardy's principle of animal exclusion in order to

　　A.give an example of one theory about the interaction of grazers and phytoplankton

　　B.defend the first theory of algal defenses against grazing

　　C.support the contention that phytoplankton numbers are controlled primarily by environmental factors

　　D.demonstrate the superiority of laboratory studies of zooplankton feeding rates to other kinds of studies of such rates

　　E.refute researchers who believed that low numbers of phytoplankton indicated the grazing effect of low numbers of zooplankton

　　It can be inferred from the passage that the "first theories" of grazer control mentioned in line 5 would have been more convincing if researchers had been able to

　　A.observe high phytoplankton numbers under natural lake conditions

　　B.discover negative correlations between algae and zooplankton numbers from their field research

　　C.understand the central importance of environmental factors in controlling the growth rates of phytoplankton

　　D.make verifiable correlations of cause and effect between zooplankton and phytoplankton numbers

　　E.invent laboratory techniques that would have allowed them to bypass their field research concerning grazer control

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