OG10_Q204,请教并列对称结构

204. In recent years cattle breeders have increasingly used crossbreeding, in part that their steers should acquire certain characteristics and partly because crossbreeding is said to provide hybrid vigor.

(A)  in part that their steers should acquire certain characteristics

(B)  in part for the acquisition of certain characteristics in their steers

(C)  partly because of their steers acquiring certain characteristics

(D)  partly because certain characteristics should be
                acquired by their steers

(E)  partly to acquire certain characteristics in their
                steers

OG解释的平行对称结构为partly to do and partly because...

两个成分一个是不定式，一个是独立的句子，怎么能对称呢？？

这个为什么可以省略？可不可以抽象出来什么情况下，什么部分能够省略。谢谢！

我的个人看法，希望能帮得上

要按照ETS的思路走，不要按着自己的思路去审视ETS，GMAT考试更多的是在考思路，语法题要按照排除的思路来看待语法点，就算是在找语法点也是有一个优先等级的，很多语法点，没有绝对的错误，也没有绝对的正确，只有相对的错误，只有比较的正确，GMAT中的绝对东东只有那么一些，所以其他语法点一定要比较。引用1stzhang大牛的话答案是比出来的，绝对不是看出来的。--强烈推荐1stzhang的三板斧，我每天做题之前都会看一遍

这道前置省略的题OG就这一题，所以不要把它作为判断等级高的语法点，而是避实就虚，就像这一题，你肯定知道首先排除A,B,C然后来比较D,E .看到D选项certain characteristics should be acquired by their steers; the steers, however, are not agents in the acquisition.看懂意思的话，这句很荒谬。一定要注意，出现被动语态的时候一定要小心，除非有确切的证据（比如，后边有提示词 by），否则是要做一个排除的错误点的，而且这个错误是要大过前置省略的错误。

某前辈NN解释--“根本上讲还是对并列理解的问题，说说容易，真正深刻的理解还是很难的，比如说形式上要并列，D看上去很美-because 同because of 的并列很奇特，但E却是形式上最对称的（从本质上看）-划线部分: 看动作的发出者是否合理有效-联想: to keep it from being stolen VS avoid being stolen/
and partly because crossbreeding is said to provide hybrid vigor.=相当于一个插入成分, it is said, ”

http://forum.chasedream.com/dispbbs.asp?BoardID=23&ID=3287&replyID=&skin=1

以后请自己搜索好吗？