单项选择题

Great as Newton was, many of his principles ______ and modified by contemporary scientists today.

A. are to challenge
B. may be challenged
C. are challenging
D. have been challenged
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单项选择题
John will have to go to work by bus as his car ______.
A. is repaired
B. is being repaired
C. is needed repairing
D. is needed repairs
单项选择题
The house had always been crowded, and with the birth of the twins the situation certainly ______.

A. has not improved
B. was not improved
C. is not improved
D. doesn’t improve
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