Final answer:
A perfectly competitive firm will find its profit-maximizing level of output where MR = MC, and for monopolists, this is not the case.
Step-by-step explanation:
A perfectly competitive firm will also find its profit-maximizing level of output where MR = MC. The key difference with a perfectly competitive firm is that in the case of perfect competition, marginal revenue is equal to price (MR =P), while for a monopolist, marginal revenue is not equal to the price, because changes in quantity of output affect the price.