___ he was out of the room,he turned back and knocked at the teacher's door again.

问题描述:

___ he was out of the room,he turned back and knocked at the teacher's door again.
A Before
B Since
C As soon as
D Unitil
说明原因

C
AD句意不通 ,B时态不符(应该是since+完成时)
这句话翻译过来大概就是他一出老师的办公室就马上回来又重敲了