(B) doesn't necessarily solve the question, and we can see this with an example. Let's say we had one species of bird with 1000 birds that flies in, and another mosquito-eating species had 999 birds fly in. If those 999 still ate a lot of mosquitoes and the 1000 didn't, it would still be puzzling that the mosquito population increased instead of decreased. Knowing the birds that eat the mosquitoes aren't the most populous species doesn't help us understand why the mosquito problem is worsening. Hope this helps! Feel free to ask any further questions you may have.