单选题 . ①In 1995, after an absence of nearly 70 years, wolves were reintroduced into Yellowstone National Park. ②During the wolf-free era, heavy browsing of aspen trees by elk populations spelled doom not only for trees themselves but for a host of other creatures dependent on them, such as beavers, whose population in Yellowstone crashed after wolves were removed. ③Without beavers to create ponds, wetland ecosystems—aquatic plants, amphibians, birds—were devastated. ④When wolves returned, grazers and browsers resumed normal patterns of behaviors, preferring safer, open areas over the dense cover and streamsides where carnivores can lurk. ⑤Keeping elk wary and on the move, wolves gave aspen and other young trees the opportunity to grow and become reestablished.

27. The passage asserts which of the following about beaver populations in Yellowstone? ______

A、 They have rebounded since the reintroduction of wolves.
B、 They were adversely affected by the feeding habits of elk population.
C、 They increased during the period when wolves disappeared from the park.
D、 They have historically had an adverse effect on the park's wetland ecosystems.
E、 They are essential to the health of the park's aspen trees.

【正确答案】 B

【答案解析】 B：对应文章第②句。
A：在④⑤两句中作者提及狼回归后对生态系统的影响，但是并没有强调海狸的数量变化一
C：狼消失的那段时间里，海狸的数量应该是下降的，选项C的表述相反。
D：是麋鹿产生了负面影响，不是海狸。
E：文章只提到了海狸依赖于白杨树，所以白杨树对海狸很重要，但没有提到海狸对白杨树的作用。