单项选择题

--This is ______ useful book, I think.-- So it is, and it's ______ unusual one.

A． a; a
B． a; an
C． an; an
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单项选择题
Usually we don't realize how important nature is to us ______ it is too late.
A． after
B． unless
C． until
单项选择题
John, get up please I think you're old enough ______ yourself, mother said.
A． to dress
B． to wear
C． to put on
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