PREP题，个人认为答案有问题

mudiduange

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
CD两个选项，C是很规范的not, but 结构；而D是are not, but are结构，有损平行。PREP笔记上说，C有they enhance reproduction or survival, but simply random by-products of other evolutionary developments的歧义，我个人认为并非如此。因此前者说的是reproduction，后者说的是by-product；二者并不会造成歧义

wudiyeah

http://forum.chasedream.com/GMAT_SC/thread-505900-1-5.html
LZ看这个解释吧