prep-2-40

40.
        (26538-!-item-!-188;#058&003184)

According to one expert, the cause of genetic irregularities in many breeds of dog is not so much that dogs are being bred for looks or to meet other narrow criteria as that the breeds have relatively few founding members.

(A) the cause of genetic irregularities in many breeds of dog is not so much that dogs are being bred for looks or to meet other narrow criteria

(B) the cause of genetic irregularities in many breeds of dog is not as much their being bred for looks or meeting other narrow criteria as much

(C) it is not so much the cause of genetic irregularities in many breeds of dog that they are being bred for looks or meeting other narrow criteria as much

(D) it is not so much that the cause of genetic irregularities in many breeds of dog is their being bred for looks or meeting other narrow criteria so much

(E) it is not so much the cause of genetic irregularities in many breeds of dog to be bred for looks or to meet other narrow criteria

之前做这道题是通过平行结构排除的

但我看prep笔记写到“C.D.E都有it表语从句用法错误”，不大明白，什么是it表语从句错误，正确的该怎么写？