which只能以疑问代词的身份（即意思为“哪个”）来引导表语从句,如：
The question is which he will choose.
若以关系代词的身份,则只能引导定语从句（一般可以用that代替,或者省略）,如：
This is the question which/that I want to ask.
题目句子的空格处,显然不能解释为“哪个”,所以不能用which.再举个相似的例子：
This is what I want to ask.——不能说 This is which I want to ask.