D. The store should close some business when sales reach N. 16. For Richard, his utility function ([) for consuming a bundle of X (sandwich) and Y (Dr. Pepper), is U - 2X3 + vY . Now suppose, a Dr. Pepper gives him a constant utility at 9. X is the number of sandwich he eats. What is marginal utility of eating the 3"d sandwich for him ? A. 5 B. 14 C. 19 D. 38 17. Which statement about time horizon in economics is correct A: In the short-run, we have only variable inputs. B. In the short-run, we usually mean the horizon is less than one year. C. In the long-run, every input is variable. D. In the long-run, producers are not able to exit the market freely. 18. Compare profit maximization of perfect competition and monopoly, which statement about marginal cost (MC), marginal revenue (MR),-average revenue (AR) and market price (P) is correct ? A. Perfect competition: MC = MR