单项选择题

Great as Newton was, many of his ideas ______ today and are being modified by the work of scientists of our time.

A．are to challenge
B．may be challenged
C．have been challenged
D．are challenging
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单项选择题
Classification is a useful ______ to the organization of knowledge in any field.
A．means
B．approach
C．mode
D．manner
单项选择题
It is no good ______ him. He is always indifferent towards other's matters.
A．to turn to
B．turning to
C．turn to
D．turned to
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