[单项选择]

Passage One
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 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 benef
A. Children
B. Young people
C. Old people
D. People of all ages