单项选择题

Trial and error ______ the source of our knowledge.

A．is
B．are
C．has been
D．have been
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单项选择题
A teacher cannot take it for granted that his students always do their homework ______.
A．as follows
B．as required
C．as they have
D．as they do
单项选择题
The police warned the residents in the area to keep their doors and windows ______ at night.
A．closed
B．closing
C．to close
D．to be closed
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