What are the most important differences between perfectly competitive markets and Unlike in perfectly competitive markets, in monopolistically competitive markets, A. firms face horizontal demand curves, and there are no barriers to entry B. firms face downward-sloping demand curves, and the products competitors sell are identical. C. firms face downward-sloping demand curves, and the products competitors sell are differentiated D. there are only a few sellers, and the products competitors sell are differentiated E. firms face downward-sloping demand arves, and there are substantial barriers to entry