求一道填空题的解答

A major tenet of the neurosciences has been that all neurons (nerve cells) in the brains of vertebrate animals are formed early in development. An adult vertebrate, it was believed, must make do with (i)___ neurons: those lost through (ii)___ or injury are not replaced, and adult learning takes place not through generation of new cells but through (iii)___ among existing ones.　
A a fixed number of      D revolution           H reproduction of older ones
　　B abundant            E disease              G modification of connections
　　C minimal               F generation           I deduction of similarities

第二空怎么选出来的，答案是D

跪求大牛们解答！！！！！！！！

A quick Google search indicates the right answer is E: disease.
see: http://gmatclub.com/forum/a-major-tenet-of-the-neurosciences-has-been-that-all-neurons-99355.html