这个是同位语从句还是定于从句

题目

这个是同位语从句还是定于从句
the school made it a rule that students should up when class begins.

答案

不可以,假设时定语从句,那么先行词是可以代入从句中,
如Who is the woman that is sweeping the floor over there.
先行词是woman,可以代入从句the woman is sweeping the floor over there.
这题的先行词是 idea,代入从句是idea is actually going on in the class room,明显不对,所以不是定语从句,是同位语从句,起补充说明的作用.