which what 引导表语从句的问题,急!

问题描述:

which what 引导表语从句的问题,急!
Coming my way from across the parking lot was ____ society would consider a bum.
为什么这里要填what而不是which,他们不是都可以引导表语从句吗?
各位大侠们帮个忙!

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.