单项选择题

It is not until recently ______ there are many different kinds of hearing defects.

A．have the scientists discovered
B．the scientists have discovered
C．that the scientists have discovered
D．that will the scientist have discovered
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单项选择题
He actually didn't see the man ______ the room, but he heard them ______ in it.
A．to enter, laugh
B．enter, to laugh
C．to enter, laughing
D．enter, laughing
单项选择题
It was when he began to make the new experiment in 1908 ______ such an ambitious plan.
A．had he
B．did he have
C．that did he have
D．that he had
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