单项选择题

I ______ since seven this morning.

A．had typed
B．typed
C．am typing
D．have been typing
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单项选择题
I could have done it for you if you______.
A．ask me to
B．asked me to
C．have asked me to
D．had asked me to
单项选择题
The greatest invention of the 20th century is ______ computer.
A．/
B．the
C．a
D．an
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