GWD5-34

casualwark11

Q34:

Many entomologists say that campaigns to eradicate the fire ant in the LACE>United StatesLACE> 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

the answer is D, but what is the "it" used in place of?

steveyangxt

形式主语，指代后面的从句或短语。