问题补充：
in the library regulations that any book borrowed be returned by due time, or the borrower will be fined．
A. What is required
B. It is required
C. As is required
D. Which is required
答案：
A. What is required
(答案→)B. It is required
C. As is required
D. Which is required
解析：考查从句关系词的选择。根据句子结构可知该句为主语从句，it 作形式主语。
==what is required in the regulations is that …………
=As is required in the regulations, …………
