The buses,_were already full,were surrounded by an angry crowd.为什么不可以用most of them.

问题描述:

The buses,_were already full,were surrounded by an angry crowd.为什么不可以用most of them.
答案most of which.请问有什么区别?

The buses were surrounded by an angry crowd是主句,___ were already full是定语从句,从句需要引导词,them只是代词,不能充当引导词