一个英语题.帮下忙.

问题描述:

一个英语题.帮下忙.
Don't read such books _____ you can't understand .
A. that B. as C. which D. what
答案B为毛不能选C?

这个定语从句的引导词用as,这是因为先行词被such所修饰的缘故.