The key difference between a monopoly and competition is:

A. Monopolies cannot earn a profit

B. In a monopoly there is some barrier to entry that allows the monopolist to set price higher than marginal cost and could earn a profit in the long-run

C. In competition, firms earn a higher profit in the long run because they have an incentive to minimize costs

D. In competition, there are barriers to entry which cause the firm to face a downward sloping demand curve