题目内容
（请给出正确答案）
Public goods are those commodities from whose enjoyment nobody can be effectively excluded
Examples of public goods are not as rare as one might expect. A flood control dam is a public good. Once the dam is built, all persons living in the area will benefit--irrespective of their own contribution to the construction cost of the dam. The same holds true for highway signs or aids to navigation. Once a lighthouse is built, no ship of any nationality can be effectively excluded from the utilization of the lighthouse for navigational purposes. National defense is another example. Even a person who voted against military expenditures or did not pay any taxes will benefit from the protection afforded.
It is no easy task to determine the social costs and social benefits associated with a public good. There is no practicable way of charging drivers for looking at highway signs, sailors for watching a lighthouse, and citizens for the security provided to them through national defense: Because the market does not provide the necessary signals, economic analysis has to be substituted for the impersonal judgement of the marketplace.
With what topic is the passage mainly concerned？
A．Mechanisms for safer navigation.
B．The economic structure of the marketplace.
C．A specific group of commodities.
D．The advantages of lowering taxes.
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