TOEFL Reading: ETS-TOEFL阅读机经 - YK53YSN5Z34IXR84L$

Which of the following can be inferred from paragraph 3 about keystone species? A. Their roles have been understood for a long time. B. They are related to a large number of ecosystems. C. They are now seen as less important in nature. D. They are the subjects of ongoing research.