TOEFL Reading: ETS-TOEFL阅读机经 - PIX35CR13892F5O6L$

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. During the period that early amphibians changed into reptiles, their skeletons were exactly the same. B. It is difficult to identify the time that early amphibians changed to reptiles because their skeletons look so much alike. C. The skeletons of some early amphibians and reptiles are so similar that it is difficult to say which are amphibians and which are reptiles. D. Early amphibians and reptiles had the same kind of skeleton at the same point in time.