[请教]one SC from Kaplan 2004 CAT1

dc

Farmers greatly increase the number of pests by providing them with a uniform food source in the form of large areas of single-crop vegetaton, but these pests can be controlled when they use synthetic, or better yet, biological pest controls.
A) but these pests can be controlled
B) but these pests are controllable
C) but they can control these pests
D) which can be controlled
E) except that can be controlled

The answer is C). Kaplan explains that "it is considered better form to use the active voice", while does not indicate anything wrong with A) & B).

My question is, in C) the use of "they" is not grammatically clear, especially there is a "them" refer to pest in the early part of the sentence. I thought it is a big no-no for GMAT that multiple "they" refer to different object in the same sentence. Is Kaplan wrong ?

chenmoon

据说从句中的代词优先指代主句主语，所以如果选择A或者B的话，会有更大的歧义，就好像说“这些宠物可以被控制，当这些宠物使用。。。的时候”。

dc

I still think the original sentence is incorrect.
"them" can only refer to "farmers" according to GMAT grammar. "pests" is in the phrase "the number of pests".

Comments ?