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
My wife couldn't sleep last night because the tap in the bathroom was ______
A．draining
B．dropping
C．spilling
D．dripping
单项选择题
Reasons for careful teaching range from preventing a child ______ to keeping a child occupied.
A．sinking behind
B．falling behind
C．drawing behind
D．crawling behind
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