题目

--Shall Mary come and play computer games? --No,__________ she has finished her homework. A. when         B. if               C. unless    D. once 答案：C 解析: 本题只要理解了句意就能很容易地选出unless。句意为：“玛丽要来打电脑游戏吗?”“不，除非她完成了作业。”

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