I won’t tell the student the answer to the math problem until he on it for more than an ho

I won’t tell the student the answer to the math problem until he on it for more than an hour.
A. has been working
B. will have worked
C. will have been working
D. had worked

(答案→)A. has been workingB. will have workedC. will have been workingD. had worked解析：正确答案A。在时间状语从句中用一般现在时代替将来时，故排除选项B和C;据时态呼应规律排除选项D;学生一直在做数学练习，直到规定时间完毕教师才公布答案。　　某个动作或过程过去已经开始，一直持续到说话时刻，有可能继续延续。因此，现在完成进行时仍带有进行时的持续性;某个动作或过程过去的过去已经开始，一直持续到过去某个时刻，有可能继续延续，用过去完成进行时。

谢谢了