单项选择题

This work costs us nothing; it's all done by ______.

A．profiteers
B．mountaineers
C．engineers
D．volunteers
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单项选择题
In the meantime, the question facing business is whether such research is ______ the costs.
A．worth
B．worth of
C．worthy
D．worthwhile
单项选择题
If you ______ the bottle and cigarettes, you'll be much healthier.
A．take off
B．keep off
C．get off.
D．set off
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