Determine if each example represents a barrier to entry or not. a. Pfizer is the only firm that is legally allowed to produce and sell Lipitor, a best-selling cholesterol drug. b. DeBeers owns nearly all of the world's diamond mines. c. Boeing already serves a large fraction of the jumbo jet market and is able to produce at a lower average cost than any potential competitors. d. Tinseltown Theaters shows almost all the most popular newly-released movies