The Main Difference Between a Monopoly and a Firm in Perfect

The main difference between a monopoly and a firm in perfect competition is:

A) the monopoly faces a perfectly elastic demand curve.
B) the firm in perfect competition sets marginal revenue equal to marginal cost in order to maximize profits.
C) the firm in perfect competition eventually faces increasing average total cost in the short run.
D) the monopoly has some control over the price of its product.