19. Which aspect of agriculture would best illustrate perfect competition? Land O'Lakes' national advertising program for its dairy products Grapefruit producers organized under the Sunkist cooperative Wheat farmers forming a cartel to restrict output and drive up price Corn farming by thousands of farmers throughout the United States envy" 20. A perfectly competitive firm's shortrun supply curve is the segment of the marginal cost curve that lies above the average total cost curve marginal cost curve that lies above the average variable cost curve average total cost curve that lies above the marginal cost curve average variable cost curve that lies above the marginal cost curve PPP'P' 21. The U.S. Postal Service (UPS) has a monopolyr on the deliver of rstclass letter mail, which is the result of a. a legal, exclusive franchise to deliver letters b. the USPS's control over the supply of workers that can deliver mail