13. The use of the word "monopoly" in the name of the market structure called "mo- nopolistic competition" refers to the fact that

a. a monopolistically competitive firm faces a downward-sloping demand curve for its differentiated product and so does a monopolist.

b. monopolistically competitive markets have free entry and exit just like a mo- nopolistic market.

c. monopolistically competitive firms charge prices equal to their marginal costs just like monopolists.

d. monopolistically competitive firms produce beyond their efficient scale and so do monopolists.