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

【正确答案】 A

【答案解析】 连词用法。犯人越狱发生在警卫发现之前，It was...before意为“(……之后)才……”，符合题意；until意为“直到……为止，直到……才”，一般不用于It was...until肯定句中。故答案为A。