It was not ___ midnight that the

It was not ___ midnight that they discovered the children were not in their beds. 
A、before
B、at
C、after
D、until
【正确答案】：D
【题目解析】：本句考查not until直到……才。本句的意思是：“直到半夜他们才发现孩子们不在床上。”A、B、C与not都没有如此搭配。