划分以下句子主谓宾以及关系代语which 的用法,此句中为什么不用what作关系代词?
问题描述：
划分以下句子主谓宾以及关系代语which 的用法,此句中为什么不用what作关系代词?
They walked in the streets for two hours carrying a sign which said,"we need two tickets."
答
主语 They
谓语walked
地点状语in the streets
时间状语for two hours
伴随状语carrying a sign (定语从句which said,"we need two tickets.")
which在定语从句中作主语,不能用what代替,但可以换成that