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
the answer is D, but what is the "it" used in place of?
The rain make it difficult for me to catch a taxi.
这个问题，我有不同的意见。这里的 it 有指代对象的，就是后面的 to spread ,指代的是这个动作或结果。同样， “The rain make it difficult for me to catch a taxi.”这里的it指代的是to catch a taxi. 我把这句话在意思不变的情况下改写一下，大家就都明白了：The rain makes catching a taxi difficult for me. 或者，Because of the rain, it is difficult for me to catch a taxi.
