Mr.Smith took interest in ____ he had seen in the history mu

Mr.Smith took interest in ____ he had seen in the history museum.A all what B all which C what
D that 请问这题目选什么?为什么?

C
宾语从句
把what he had seen in the history museum 作为宾语——他在历史博物馆看到的东西.
再问： 请问前面为什么不能加all？
再答： 没有这种用法 你可以填all that表示他在历史博物馆看到的所有东西，此时that是关联词，构成定语从句，he had seen in the history museum修饰all。all是不定代词，定语从句中只能用that作关联词，所以B也不对。但是没有all that这个选项。