The U.S. Postal Service has a monopoly on non-urgent First Class Mail. Pfizer Inc. makes LIPITOR, a prescription drug that lowers cholesterol. Cox Communications is the sole provider of cable television service in some parts of San Diego. 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? Explain your answers.