第19题: [单项选择]A man escaped from the prison last night. It is a long time ______ the guards discovered what had happened. A. before B. until C. since D. when 参考答案:A 答案解析:本句的意思是:昨夜有人从监狱逃走,过了很长时间警卫人员才发觉(有人逃走)。意即“在警卫人员发觉有人逃走之前,已过了很长时间。”再如:New ideas sometimes have to wait f