We can’t do two things .A. all the time B. on timeC. at timesD. at a time

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A. all the time
B. on time
C. at times
(答案→)D. at a time
解析：此题重点考查短语辨析，all the time一直；on time 准时、按时；at times有时、偶尔；at a time 一次，根据句意可知，我们不能一次做两件事，故选D