单项选择题

Cement was seldom Used in buildings in the Middle Ages.

A. crudely
B. rarely
C. originally
D. occasionally
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The most crucial problem any economic system faces is how to use its scarce resources.
A. puzzling
B. difficult
C. terrifying
D. urgent
单项选择题
Techniques to employ the energy of the sun are being developed.
A. convert
B. store
C. use
D. receive
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