问题
选择题
— ____ sweep the classroom yesterday？
—I was playing tennis with my friend．
A．Why not
B．Why didn't
C．Why don't
D．Why didn't you
答案
答案：D
选项B、C错误，因为句子缺主语；选项A错，因为“Why not（+动词原形）”这一结构通常不用于过去（即不与过去时间连用）。
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