TOEFL Reading: ETS-TOEFL阅读机经 - 46KGR4L9M2E1W7TNL$

Which of the following can be inferred from paragraph 2 about arthropods? A. There are fewer of them than there are of vertebrates. B. Their ancestors had a selective advantage over the ancestors of vertebrates. C. Their ancestors once possessed internal skeletons. D. Their body features prevent them from becoming large organisms.