求助：XDF-section20-Q7 (新东方蓝皮;大全没有)

Q7

By the same techniques used for genetically enhancing plants, making them disease- or pest-resistant, researchers have been able to increase the amount of protein in potatoes sweet potatoes, and tobacco.

A. By the same techniques used for genetically enhancing plants, making them

B. With the same techniques to genetically enhance plants, so that they are

C. Employing the same techniques used to genetically enhance plants, so that they are

D. Employing the same techniques to genetically enhance plants, which makes them

E. Employing the same techniques for genetically enhancing plants that make them

答案 C

表示目的用to do=>排除AE

“which makes”应为复数=>排除D

剩下CB，我选了B，我觉得C中又有employing又有used语意重复了。而B用with引导介宾与逻辑主语researchers也是一致的呀。不明白为什么答案选C，难道是B中的so that有问题吗？请牛牛指点

谢谢解答。不过还是不太明白。

主语是researchers为什么不可以用B项with介宾短语作状语，而用doing放句首做伴随呢，表达的意思上有什么不同么？能否详细说明？

而且，我觉得C中又有employing又有used，这样难道语意不重复么？