The profit$-$maximization problem for a monopolist differs from that of a competitive firm in which of the following ways?

a$.$ A competitive firm maximizes profit at the point where marginal revenue equals marginal cost; a monopolist maximizes profit at the point where marginal revenue exceeds marginal cost.

b$.$ For a competitive firm, marginal revenue at the profit$-$maximizing level of output is equal to marginal revenue at all other levels of output; for a monopolist, marginal revenue at the profit$-$maximizing level of output is smaller than it is for larger levels of output.

c$.$ For a profit$-$maximizing competitive firm, thinking at the margin is much more important than it is for a profl$-$maximizing monopolist.

d$.$ A competitive firm maximizes profit at the point where average revenue equals marginal cost; a monopolist maximizes profit at the point where average revenue exceeds marginal cost.