The United States Postal Service has a monopoly on non-urgent First Class Mail. Esperion Therapeutics makes Nexletol, a prescription drug that lowers cholesterol. SkyCity is the sole restaurant that provides 360-degree views of Seattle. Are any of these firms protected by a barrier to entry? Do any of these firms produce a good or service that has a substitute? Might any of them be able to profit from price discrimination? Part 2 The United States Postal Service has _______ barrier to entry. Esperion Therapeutics has _______ barrier to entry