单项选择题

It was not until 1920 ________ regular radio broadcasts began.

A．while
B．which
C．that
D．since
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单项选择题
Not only I but also Jane and Mary ________ tired of having one exam after another.
A．is
B．are
C．am
D．be
单项选择题
When Jack arrived, he learned Mary ________ for almost an hour.
A．had gone
B．had set off
C．had left
D．had been away
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