It is like when you take a medicine you don't need, it becomes useless when you need it because your body can fight it off. So, I would have to say that the pesticides have been used on the bugs so much that they can finally fight it off. I really hoped I helped :D
Answer:
Because of the fact that it was very Deep and heavily polluted.
Explanation:
Actual main channel depths along the river vary; with a least depth of 24' or less at a few locations at the channel edges to a maximum depth of 108' being found mid-channel at the southern approach to Hell Gate.
Due to heavy pollution, the East River is dangerous to people who fall in or attempt to swim in it, although as of mid-2007 the water was cleaner than it had been in decades.
The answer for this question is B.
Hope this helped!!
