单选题     Public goods are those commodities from whose enjoyment nobody can be effectively excluded. Everybody is free to enjoy the benefits of these commodities, and one person' s utilization does not reduce the possibilities of anybody else' s enjoying the same good.
    Examples of public goods are not as rare as one might expect. A flood control dam is public goods. 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 light house 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 public goods. 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.

单选题     With what topic is the passage mainly concerned?

单选题     Which of the following would NOT be an example of public goods as described in the passage?

单选题     According to the passage, finding out the social costs of a public good is a______.

单选题     Which of the following statements best describes the organization of the first two paragraphs?