求高手指点～一道百思不得其解的语法题目

Seven7zxh

GWD题目
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 aresimply 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
答案选择D，但是这个答案里面的连词but前面已经有了逗号隔开，这时候but 和are之间不是应该加上主语么？为什么不加they呢？



还有一个今天模考到的prep 题目
Since the start of the space age , more and more littering has occurred in orbits near Earth, often because the international discarding of lens caps, packing material fuel tanks ,and payload covers.



B.orbits near Earth have become more and more littered often from

C.there have been more and more littering orbits near Earth, often because of

答案选择B。。。＝ ＝晕，是我一开始就排除掉的答案。
from有因为的意思么？

如果有高手明白的，麻烦指点一下哈～马上就要考试了呵呵多谢多谢！