— 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
网友回答
A. Why not
B. Why didn't
C. Why don't
(答案→)D. Why didn't you
解析：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）。