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
Cement was seldom used in buildings of the Middle Ages.()

A. slightly
B. rarely
C. originally
D. occasionally
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