请问d选项的it在这里指代的是什么？

请问d选项的it在这里指代的是什么？

为什么不能看做which 指带前面的这件事情呢？

d选项的making的主语应该是哪个？这个选项是什么语法现象？ 为什么能这么用？

thus是副词，副词不是加逗号连接两个完整句子吗？那thus也不可以啊

老师你好，这道题不能用which是不是因为指代不明确呢？which就近指代到natural enemies不符合句子逻辑，也可能往前指代到chemicals、campaigns，所以要排除AB。然后CD的it指代什么呢？C选项them是不是指代对象不明，没有D好呢？E选项为什么错呢？我读的特别不通顺，直接排除了，但是说不出哪里不对。想请王文静老师解答一下，谢谢！

because和thus不会重复吗？

因为because。。。 thus。。。。。，我上来就把CDE排除了。请老师讲讲排除方法。

The Sports Medicine Programs of the Olympic Training Center, a complex where final tryouts are held for athletes representing the United States in the Olympics, is geared toward enhancing athletes' performance and toward their preparation for international competition. (A) is geared toward enhancing athletes' performance and toward their preparation (B) is geared to enhance the performance of athletes and to prepare them (C) are geared to enhance athletes' performance and their preparation (D) are geared toward the enhancement of athletes' performance and toward preparing them (E) are geared toward enhancing the performance of athletes and preparing them 答案是E， 我选C， 语法笔记里说C错是因为to enhance和their preparation不平行，我想问为什么不能理解为athletes' performance 和their preparation平行呢？