❓ 题目 According to paragraph 2, which of the following statements about the keystone species concept is TRUE?
- A. A) The concept was first proposed by Paine based on experiments involving beavers.
- B. B) Keystone effects are limited to predation and competition for space.
- C. C) The concept has been expanded to include non-predatory mechanisms and various ecological scales.
- D. D) Ecologists can easily predict keystone species without field experiments.