单项选择题

The most crucial problem any economic system faces is how to use its scarce resources.

A．puzzling
B．difficult
C．terrifying
D．urgent
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单项选择题
The substance can be added to gasoline to accelerate the speed of automobiles.
A．quicken
B．shorten
C．loosen
D．enlarge
单项选择题
He invested a considerable amount of money in the project.
A．immense
B．positive
C．powerful
D．realistic
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