[求助]GWD5-Q34,it指代

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.

A.      which made it easier for them

B.      which makes it easier for it

C.      thus making it easier for them

D.      thus making it easier for the ant

E.       thereby, it was made easier for the
ant

答案是D，虽然作对了，可是不明白D的it指代什么？