❓ 题目 According to the passage, which of the following is a limitation of the keystone species concept?
- A. A) It only applies to top predators, not herbivores or mutualists.
- B. B) The effects of a keystone species are always consistent across different ecosystems.
- C. C) It is difficult to measure what constitutes a 'disproportionate' effect.
- D. D) The concept was introduced before any experimental evidence was available.