单项选择题
These computer programs are a ______ to me.
A．problem
B．difficulty
C．puzzle
D．question
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单项选择题
The fishing boat sank below the waves, and was seen ______.
A．any more
B．not more
C．no more
D．not any
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单项选择题
Most environmental problems exist because adequate measures for preventing them ______ taken in the past.
A．was not
B．were not being
C．were not
D．was no to be
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