24. Which of the following statement(s) is/are TRUE? I. Because monopolistically competitive firms sell differentiated products, their demand curves are downward sloping. II. Monopolistically competitive firms earn above-normal profits because of high entry barriers. III. As firms enter a monopolistically competitive industry, the demand curves of the existing firms shift down and to the left. A) I and III only B) I and II only C) II only D) III only