Did he come back early?

问题描述:

Did he come back early?
Yes.It was not yet eight o'clock ___ he arrived home
A before B when
选B 为什么不选A

when引导的时间状语从句表示的是一个时间点,主句可以是一个确切的时间,而before则不可.
如:It was just ten when he went to bed last night.