Being a teacher always encouraging students to think for themselves, I won't tell them answers until they on a math problem for more than an hour.
A. have worked
B. will have worked
C. will have been working
D. had worked
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(答案→)A. have worked
B. will have worked
C. will have been working
D. had worked
解析：考查时态：句意：做老师总是鼓励学生独立思考，直到他们这道数学题算了一个多小时我才告诉他们答案。主句是一般将来时，从句用一般现在时或现在完成时代替将来时，或将来完成时。选A。