If a positive externality is to be taken full advantage of, the:
A-consumer of the good should receive a subsidy equal to the marginal cost imposed on third parties that results from production (or consumption) of the good.
B-consumer of the good should pay a tax equal to the marginal benefit to third parties that results from production (or consumption) of the good.
C-producers' marginal costs should be increased by an amount equal to the marginal benefit to third parties that results from production of the good.
D- producers' marginal costs should be decreased by an amount equal to the marginal cost imposed on third parties that results from production of the good.