单项选择题

The Egyptians ______ copper as early as 5,000 years before Christ.

A．they were using
B．having used
C．may have used
D．using
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单项选择题
Americans have been criticized for placing too much ______ on being on time.
A．emphasis
B．activity
C．bother
D．assistance
单项选择题
That teacher is very good ______ children.
A．over
B．at
C．on
D．with
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