快考试了，有2道题一直纠结，求NN指点！谢谢！

azhongzhong

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
我选的C,为何要选D,在but后加are，不是not...but的结构吗
81.  Recently physicians have determined that stomach
ulcers are not caused by stress, alcohol, or rich food
but a bacterium that dwells in the mucous lining of th
stomach.
(A)  not caused by stress, alcohol, or rich foods, but
(B)  not caused by stress, alcohol, or rich foods,
but are by
(C)  caused not by stress, alcohol, or rich foods,
but by
(D)  caused not by stress, alcohol, and rich foods,
but
(E)caused not by stress, alcohol, and rich foods,
此题答案C，OG的解释是not...but就够了，不用补出。
这两道题有矛盾啊
现在头晕了，到底何时要补出，何时不补啊？求大牛解释，谢谢！

ivyxiaoxiao

恳请NN帮忙解释下这两道题哦~