accompanying figure. At quantities less than 50 doughnuts per day The graph plots quantity of doughnuts per day versus price in dollars per doughnut. A dashed curve and a curve intersect at (50, 0.10). Select one: a. the return to the firm's variable factors of production must be increasing. b. average cost is declining because marginal cost is increasing. c. average cost is declining because marginal cost is less than average cost. d. marginal cost is declining