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 workedB.will have workedC.will have been workingD.had worked
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A解析试题分析：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