单项选择题

______ you understand this rule, you'll have no further difficulty.

A. Once
B. Unless
C. As
D. Until
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单项选择题
About ______ of these rooms ______ empty.
A. two third, are
B. two thirds, are
C. two thirds, is
D. two third, is
单项选择题
I was so familiar with her that I recognized her voice ______ I picked up the phone.
A. the moment
B. after
C. before
D. while
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