You have to wait another two hours,( ) the doctor will have finished his operation.
问题描述：
You have to wait another two hours,( ) the doctor will have finished his operation.
A:before B:until C:when D:as
不大明白,望详解,
答
你还要再等两个小时,那时医生就会完成手术了.
一般人会认为是not until的句型,不过这里前面没有not
这里的when相当于by then
说话的人并不是告诉（患者）要等到医生手术后再如何,后面无非是一个解释,主要的内容是前半句,你不宁再等上两个小时,到时候医生就会做完手术来给你看病.
如果要用until
The doctor will not be available until he finishes his operation in about two hours.
不过,还是避免用这样的题来让学生摸不着头脑为好,估计你的老师也很可能会选择until的
再有一个更清楚的解释,until后面不用完成时态