单项选择题

They regard ______ as their duty to provide the best service for their customers.

A．this
B．what
C．it
D．that
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单项选择题
Although he did not feel well, he insisted going there together with us.
A．to
B．on
C．at
D．for
单项选择题
I'll ask Mr. Smith to ring you up ______ he comes back to the office.
A．when
B．where
C．because
D．although
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