A man escaped from the prison last night. It was a long time ________ the guards discovered what had happened.

A.before

B.until

C.since

D.when

答案及解析： C

解析：

本题要突破定向思维，不是It was a long time before句型，注意空格后面的时态是过去完成时，前面是过去时，因此从时间概念上看应该是since．