The teacher won't tell the students the answer to the difficult problem until they _______on it for more than an hour.

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1. (2020高二上·漠河期末) The teacher won't tell the students the answer to the difficult problem until they _______on it for more than an hour.

A . have been working B . will have worked C . will have been working D . had worked

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备考2022年高考英语一轮复习15：时态辨析