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—＿＿（36）＿＿ their own contribution to the construction cost of the dam. The same ＿＿（37）＿＿ 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_＿＿（38）＿＿ military expenditures or did not pay any taxes will benefit from the protection provided. It is no easy task to determine the social costs and social benefits ＿＿（39）＿＿ 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 ＿＿（40）＿＿ for the impersonal judgment of the marketplace.