Sentence Correction

Hey Guy, I have some trouble seeing why I can't use which in the answer for this question.

Many entomologists say that campaigns to eradicate the fire ant in the United States have failed because the chemicals that were used were effective only in wiping out the ant's natural enemies, which made it easier for them to spread.

"¢which made it easier for them
"¢which makes it easier for it
"¢thus making it easier for them
"¢thus making it easier for the ant
"¢thereby, it was made easier for the ant

Can anyone tell me why?
Many thanks
Lukas

"which" is wrong here because it will refer to "ant's natural enemies"
now is it the "ant's natural enemies" that made the spreading of ant's easier ? --->the answer is no

it is in fact because the chemicals that were used were effective only in wiping out the ant's natural enemies that made the spreading of ant's easier !!
so the answer should be D