What will be an ideal response?
ANSWER:
A perfectly competitive firm sells its product at a price that equals the opportunity cost, or the marginal cost, to society of producing one more unit of the product. Because the price that consumers are willing to pay for the last unit of the good is the marginal benefit to them, the pricing outcome of a competitive firm implies that the marginal benefit to consumers equals the marginal cost to society of producing the last unit. This outcome is efficient because it is impossible to increase the output of any good without lowering the value of the total output produced in the economy.