单项选择题

The solution cannot be determined _____the problem is identified.

A． if
B． when
C． until
D． which
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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
单项选择题
Some managers wouldn't let the assistants ______ early yesterday.
A． leave
B． leaves
C． leaving
D． left
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