When did he leave the classroom?— He left ----- you turned back to write on the blackboard.A the minute B.the time C.until D.before为什么不能选择before

When did he leave the classroom?— He left ----- you turned back to write on the blackboard.
A the minute B.the time C.until D.before
为什么不能选择before

应该是 转过身之后离开,之前离开的话 老师又不是瞎子,应该知道的,就不用问了!