单项选择题

He has made another discovery, ______ of great importance to science.

A．which I think it is
B．of which I think it is
C．which I think is
D．I think which is
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单项选择题
Although he was ill, ______ he went on working.
A．but
B．yet
C．and
D．while
单项选择题
--Who are you going to have this letter ______ for you--My secretary.
A．type
B．typed
C．been typed
D．been typing
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