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
We desire that the tour guide ______ us immediately of any change in plans.
A．has informed
B．informed
C．informs
D．inform
单项选择题
A new technique ______ , the gross output increased by 20 percent.
A．working out
B．having worked out
C．having been worked out
D．to have been worked out
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