关于用逗号把连词隔开重不重复主语的问题！。。。

gracezz

74. (28503-!-item-!-188;#058&003635)
Paleontologist Stephen Jay Gould has argued that many biological traits are not the products of
natural selection, favored due to their enhancement of reproduction or survival, but that they are
simply random by-products of other evolutionary developments.
(A) due to their enhancement of reproduction or survival, but that they are
(B) due to the reproduction or survival they enhance, but they are
(C) because they enhance reproduction or survival, but
(D) because they enhance reproduction or survival, but are
(E) because of enhancing reproduction or survival, but are

答案是D，但是prep语法笔记里面有说：
1当用逗号把连词隔开的时候，连词就要连接两个句子，就要分别有主谓语。
2但是有一个省略原则是当前一个分句有be 或者become的时候，后面的be 和become也要省掉，相应重复的名词等也要省掉。

所以这个题是适应第2条有BE动词可以省，但是选择D不省是因为可能产生enhance...but...的歧意。

请问我这样理解对吗？

shanshanss

lz，这道题在prep讨论区里面已经讨论过很久勒~~应该是这样理解的，不过我自己也没太搞懂，所以推荐你可以去看看：）