The correct answer is: D. None of the above
In perfect competition, firms make zero economic profits in the long run. In monopoly, firms make economic profits in the long run. In oligopoly, firms can make economic profits in the long run, but they may also make zero economic profits or even losses in the long run.
Perfect competition is a market structure in which there are a large number of firms selling identical products. Firms in perfect competition have no market power and must accept the market price. In the long run, firms in perfect competition will make zero economic profits. This is because firms will enter the market if there are economic profits to be made, and firms will exit the market if there are economic losses to be made. As firms enter or exit the market, the market price will adjust until economic profits are zero.
Monopoly is a market structure in which there is only one firm selling a good or service. The monopolist has market power and can set the price of its product. In the long run, the monopolist will make economic profits. This is because the monopolist can charge a price that is above the marginal cost of production.
Oligopoly is a market structure in which there are a small number of firms selling a good or service. The firms in an oligopoly have some market power, but they do not have as much market power as a monopolist. In the long run, the firms in an oligopoly may make economic profits, zero economic profits, or even losses. This depends on the nature of the competition between the firms in the oligopoly.
In conclusion, the correct answer is: D. None of the above