21. The word “ultimately” in line 8 is closest in meaning to (A) exceptionally (B) dramatically (C) eventually (D) unfortunately

25. Why is “ plankton” mentioned in line 17? (A) To demonstrate the interdependence of different species (B) To emphasize the importance of food resources in preventing mass extinction. (C) To illustrate a comparison between organisms that live on the land and those that live in the ocean (D) To point out that certain species could never become extinct.