单项选择题

I was listening to the music ______ there was a knock at the door.

A． when
B． while
C． because
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单项选择题
You don't need to describe (描述) her. I ______ her several times.
A． met
B． have met
C． meet
单项选择题
______ is the population of Canada
A． How many
B． What
C． Which
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