Although schistosomiasis is not often fatal,it is so debilitating that it has become an economic drain on many developing countries.

(A)it is so debilitating that it has become an economic

(C)so debilitating is it as to become an economic 答案A

请问这里为什么不能选C中的so as to

C：1，"is it" wordy

2 , "so...as to" awkward

2的原因在于使用as to do 结构不能表现出 that it has become 的意思，曲解了原意。