10. A public good is nonexcludable and nonrival in consumption. In most cases a public good must be supplied by the government. The marginal social benefit of a public good is equal to the sum of the individual marginal benefits to each consumer. The efficient quantity of a public good is the quantity at which marginal social benefit equals the marginal cost of providing the good. Like a positive externality, marginal social benefit is greater than any one individual’s marginal benefit, so no individual is willing to provide the efficient quantity.