阅读原文第三段：

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.

4.The discussion concerning population in the third paragraph serves primarily to

[A] demonstrate the difficulties ecologists face in studying density-dependent factors limiting population growth.

[B] advocate more rigorous study of density-dependent factors in population growth.

[C] prove that the death rates of any population are never entirely density-independent.

[D] underline the importance of even small density-dependent factors in regulating long-term population densities.

「答案」D

「考点解析」本题是一道归纳推导题。题干已明确指出该题的答案信息来源在第三段。第三段主要陈述“density-dependent factors”的作用，过仔细阅读第三段，尤其是第三段的最后三句话，考生可以得知本文作者在第三段是在强调“density-dependent factors”的作用。可见D是本题的正确选项。考生在理解原文时一定要注意掌握归纳推导的能力。