题目内容：

关于it的用法
1:Someone is knocking at the door .Go and see_____.
为什么选：who it is 而不是who is it
4:--why don't we take a little break?
----Didn't we just have ___?
为什么是用one 不用it?

优质解答

Go and see who is it .带疑问的语气
Didn't we just have one?one 指代take a little break .而it 不能