请教高手

kun861010

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




why is d? i am totally lost.

moonlolita

AB which指代歧义，C them指代歧义，E，是不是副词做了连词？