有读者问： 有这样一道选择题：
(1)What was it_________he wanted?
A. that B. which C. what D. as
答案是A，为什么B和D不对？ 三者有什么区别？
注意：先行词如是代词it，其后的关系代词一般皆应用that，故上述句（1）的答案应是A，而不是B或D。再如：
(2)What is it that makes you cry so?
(3)It is his insolence that I object to.
为何用that而不用which？
