which只能以疑問代詞的身份（即意思為“哪個”）來引導(dǎo)表語從句,如：
The question is which he will choose.
若以關(guān)系代詞的身份,則只能引導(dǎo)定語從句（一般可以用that代替,或者省略）,如：
This is the question which/that I want to ask.
題目句子的空格處,顯然不能解釋為“哪個”,所以不能用which.再舉個相似的例子：
This is what I want to ask.——不能說 This is which I want to ask.