Sentence Correction

Farmers greatly increase the number of pests by providing them with a uniform food source in the form of large areas of single-crop vegetation, but these pests can be controlled when they use synthetic, or better yet, biological pest controls.

(1) but these pests can be controlled
(2) but these pests are controllable
(3) but they can control these pests
(4) which can be controlled
(5) except that can be controlled

Farmers greatly increase the number of pests by providing them with a uniform food source in the form of large areas of single-crop vegetation, but these pests can be controlled when they use synthetic, or better yet, biological pest controls.

(1) but these pests can be controlled
(2) but these pests are controllable
(3) but they can control these pests
(4) which can be controlled
(5) except that can be controlled

imo c

option C is the best, because 'they' after underline sentence refers properly to farmers.

Hi everyone,

Not convinced why "C" is the answer here.
I actually picked "A", but according to the explanation in the attached picture it is the "passive voice" which the GMAT apparently does not favor.
Using that logic, option "C" is picked.
However, option "C" does not strike me as a good option, since the use of the word "they" could be ambiguous, erroneously referring to the vegetation that can control the pests.

Could anyone share your views on these matters? Thanks.