阅读原文二三段：

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 population 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.

5. In the text， the author does all of the following EXCEPT

[A] cite the views of other biologists.

[B] define a basic problem that the text addresses.

[C] present conceptual categories used by other biologists.

[D] describe the results of a particular study.

「答案」D

「考点解析」本题是一道写作手法题。这是一道比较难的题目，旨在考察考生的语言功底。本文作者在论述自己的观点时运用了各种写作手段。选项A、B、C中所涉及的内容分别在段、第三段和第二段中出现。考生一定要对原文各种写作手段的认识。