单项选择题

The ______ of human knowledge are being pushed further.

A．boundaries
B．borders
C．limits
D．edges
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单项选择题
She ______ a strong and positive influence over her friends.
A．draws
B．exerts
C．ensures
D．expends
单项选择题
Since you didn't go over the lessons before the exam, you can find no ______ for your failure.
A．explanation
B．excuse
C．exception
D．expection
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