GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - A5695SS6JA0BUH2YJ

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.