15. A monopoly firm is different from a competitive firm in that: A. there are many substitutes for a monopolist's product while there are no substitutes for a competitive firm's product. B. a monopolist's demand curve is perfectly inelastic while a competitive firm's demand curve is perfectly elastic. C. a monopolist can influence market price while a competitive firm cannot. D. a competitive firm has a U-shaped average cost curve while a monopolist does not