GWD5 34 求教

GWD5-Q34:

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.

1. which made it easier for them
2. which makes it easier for it
3. thus making it easier for them
4. thus making it easier for the ant
5. thereby, it was made easier for the ant

选D ，请问一个弱弱的问题，it是代谁的？

谢谢

thus making it easier for the ant to spread.

这里的it是指to spread

请指教

是啦

没注意到讨论汇总里有这个题目

谢谢MM热心回复