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

问题补充：

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：

(答案→)A. has been working

B. will have worked

C. will have been working

D. had worked

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