请教一个not,but平行的题目

liuyinghong

Paleontologist Stephen Jay has argued that many biological traits are not the producs of natural selection, favored due to their enhancement of reduction or survival, but that they are simply random by-products of other evolutionary developments.

A  
B  due to their production 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

答案选D，我觉得C更平行啊， sth. are not A,but B
D不就成了sth. are not A, but areB
请教

victoria820

but为连词,后接完整的句子,如果没有are,那but后面的句子的谓语动词是哪个呢?

xiao19870205

有道理

haileywang

C会造成一个歧义，变成了because they enhance reproduction or survival,but simply random by-products of other evolutionary developments.这样的话，就改变原意了。
而D是更清楚的，because 是一个插入语。
not A,but B 不是固定的。but are B也是可以的，主要看没有are 会不会造成歧义。
我记得这题有讨论帖里讲到的，具体可以搜索下。我讲的比较粗糙