单项选择题

Our company ______ Metro Messenger Service since 1998.

A． use
B． used
C． had used
D． has been using
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单项选择题
The solution cannot be determined _____the problem is identified.
A． if
B． when
C． until
D． which
单项选择题
Anew collection of programs______in the conference room.
A． are presenting
B． are presented
C． present
D． is being presented
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