问题补充：
I won’t tell the student the answer to the math problem until heon it for more than an hour.
A. has been working
B. will have worked
C. will have been working
D. had worked
答案：
(答案→)A. has been working
B. will have worked
C. will have been working
D. had worked
解析：考查时态：句意：我不会告诉学生数学题目的答案，直到他算了一个多小时。首先，由于until引导的是时间状语从句，根据“主将从现”原则，当主句为将来时态时，时间状语从句不能直接用将来时态来表示将来意义，据此可排除B和C；而选项D为过去完成时，也不能选，因为“过去完成时”必须以“过去”为支点，表示“过去的过去”，但此句并没有过去时态这一支点，故此选项也被排除，这里用现在完成进行时代替将来完成进行时，选A。
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