Policy making to remedy market failure requires the government to estimate values such as marginal private costs, marginal private benefits, marginal external costs, and marginal external benefits. Indicate which of these values must be estimated correctly in order to allocate resources efficiently using each of the following approaches.

a. Taxes used to correct for a negative externality

b. Subsidies used to correct for a positive externality

c. A government provision of public goods that does not cause externalities