Is that the reason_________you are in favour of the proposal?A.which B.what C.why D.for that

题目

Is that the reason_________you are in favour of the proposal?A.which B.what C.why D.for that
我查了一下百度,答案都是C.why
为什么呢?
我在百度上看到你给另一个人讲解时说道：thatwhich后面句子成分完整
why后面是一个对其前句的补充.
那么这么说的话,这句句子不应该选A么?不过我又读why比较顺口-_-|||

答案