What did people use for building materials before?
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参考答案:
举一反三
【单选题】It is commonly argued that national defense is a public good. Nevertheless, the weapons used by the U.S. military are produced by private firms. We can conclude that
A.
resources would be used more efficiently if the government produced the weapons.
B.
resources would be used more efficiently if private firms provided national defense.
C.
weapons are rival in consumption and excludable, but national defense is not rival in consumption and not excludable.
D.
national defense is rival in consumption and excludable, but weapons are not rival in consumption and not excludable.