❓ 题目 According to the passage, what is a key characteristic that defines a keystone species?
- A. A) It must be a top predator at the apex of the food chain.
- B. B) It has an ecological impact disproportionately large relative to its abundance.
- C. C) It always occupies the highest trophic level in the ecosystem.
- D. D) It primarily regulates ecosystems through bottom-up forces.