21. A peculiar feature of the embryonic mammalian circulatory system is that in the area of the heart the cells adhere to one another, beating in unison and adopting specialized orientations exclusive of one another.
(A) beating in unison and adopting
(B) they beat in unison while adopting
(C) beat in unison, and adopt
(D) beating in unison yet adopting
(E) even though they beat in unison and adopt
选项C为何不可？ 思路：主语：CELLS， 谓语三个动词并列：adhere....., beat.., and adopt....作者: himba 时间: 2004-12-27 20:37
因为adopt ...exclusive跟前面两个动作adhere和beat in unison表示相反的意思，所以要表达转折的意思，因此C不能用and而要用but，D正确。
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