Who can think of a situation_______this word can be used?

问题描述:

Who can think of a situation_______this word can be used?
为什这道题用where而不用that,语法讲解里不是说如果当主句的主语是疑问词who或which时,那定语从句的疑问代词必须用that么?

在这个定语从句中,从句this word can be used已经不缺成分了,所以不可以填which who 等,最重要的是,当先行词是situation,condition,position,stage,等这些词时,引导词要用where