someoneknocked()thedoorwhilei()myhouseworka.at,weredoingb.at,wasdoingc.on,wasdoingd.on,weredoing
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题目内容：
someone knocked( )the door while i( )my housework
a.at,were doing
b.at,was doing
c.on,was doing
d.on,were doing
优质解答
B
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