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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Techniques to employ the energy of the sun are being developed.
A. convert
B. store
C. use
D. receive
单项选择题
The government is debating the education laws.
A. discussing
B. defeating
C. delaying
D. declining
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