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

Thus ，thereby在这里和because连用，没有语义重复的错误吗？就好像because。。。。。so一样？
做题的时候上来就派出了B C D，结果发现没有对的,翻了原来的讨论稿，没见讨论过这个问题。