.We will learn how to operate the machine .A. at a time B. at one timeC. at the timeD. in

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A. at a time
B. at one time
C. at the time
(答案→)D. in time
解析：考查短语辨析。at a time每次，at one time 曾经，in time 及时，准时，句意：我们将学会如何准时操作机器。所以选D。