诚心请教：新GRE 36套 exer23 7 8

grewalker · 发表于 2012-8-4 11:57:18

As Gilbert White, Darwin, and others observed long ago, all species appear to have the innate capacity to increase their numbers from generation to generation. The task for ecologists is to untangle the environmental and biological factors that hold this intrinsic capacity for population growth in check over the long run. The great variety of dynamic behaviors exhibited by different populations makes this task more difficult: some populations remain roughly constant from year to year; others exhibit regular cycles of abundance and scarcity; still others vary wildly, with outbreaks and crashes that are in some cases plainly correlated with the weather, and in other cases not.
To impose some order on this kaleidoscope of patterns, one school of thought proposes dividing populations into two groups. These ecologists posit that the relatively steady populations have "density- dependent" growth parameters; that is, rates of birth, death, and migration which depend strongly on population density. The highly varying populations have "density-independent" growth parameters, with vital, rates buffeted by environmental events; these rates fluctuate in a way that is wholly independent of population density.
This dichotomy has its uses, but it can cause problems if taken too literally. For one thing, no population can be driven entirely by density-independent factors all the time. No matter how severely or unpredictably birth, death and migration rates may be fluctuating around their long-term averages, if there were no density-dependent effects, the population would, in the long run, either increase or decrease without bound (barring a miracle by which gains and losses canceled exactly). Put another way, it may be that on average 99 percent of all deaths in a population arise from density-independent causes, and only one percent from factors varying with density. The factors making up the one percent may seem unimportant, and their cause may be correspondingly hard to determine. Yet, whether recognized or not, they will usually determine the long-term average population density. In order to understand the nature of the ecologist's investigation, we may think of the density-dependent effects on growth parameters as the "signal" ecologists are trying to isolate and interpret, one that tends to make the population increase from relatively low values or decrease from relatively high ones, while the density- independent effects act to produce "noise" in the population dynamics. For populations that remain relatively constant, or that oscillate around repeated cycles, the signal can be fairly easily characterized and its effects described, even though the causative biological mechanism may remain unknown. For irregularly fluctuating populations, we are likely to have too few observations to have any hope of extracting the signal from the overwhelming noise. But it now seems clear that all populations are regulated by a mixture of density- dependent and density-independent effects in varying proportions.

7.The author of the passage is primarily concerned with
(A) discussing two categories of factors that control
population growth and assessing their relative
importance
(B) describing how growth rates in natural populations
fluctuate over time and explaining why these
changes occur
(C) proposing a hypothesis concerning population
sizes and suggesting ways to test it
(D) posing a fundamental question about environ-
mental factors in population growth and
presenting some currently accepted answers
(E) refuting a commonly accepted theory about
population density and offering a new alter-
native

8. It can be inferred from the passage that the author
considers the dichotomy discussed in the second
paragraph to be
(A) applicable only to erratically fluctuating
populations
(B) useful, but only if its limitations are recognized
(C) dangerously misleading in most circumstances
(D) a complete and sufficient way to account for
observed phenomena
(E) conceptually valid, but too confusing to apply
on a practical basis

答案是A B。
但是我觉得第八题的A为什么错，还有B的 useful , but only if its limitations are recognized 但是只要他的限制被识别旧有用。怎么感觉这么怪？直接说“有用，但是有限制”不好吗？
再者，第七题的D和E怎么排除？恕我愚钝，这不是解释问题型+新老观点对比型吗？

麻烦高手花点心思看一下，回答小弟的疑惑。谢谢~~

grewalker · 发表于 2012-8-4 14:10:18

人工置顶。。

诚心请教：新GRE 36套 exer23 7 8

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