问题补充：
When Mary came, I on the piano.
A. is playing
B. plays
C. was playing
D. played
答案：
A. is playing
B. plays
(答案→)C. was playing
D. played
解析：考查时态：句意：当玛丽来的时候，我正在弹钢琴。从句意看，弹钢琴是过去某个时间正在发生，用过去进行时，选C。
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