Wilbur's Bean Emporium serves barbeque sandwiches over the lunch hour. The marginal cost of the 50th barbeque sandwich is $1.50. The average total cost of the 49th sandwich is $1.75. For Wilbur's Bean Emporium, __________ when output is 50 sandwiches. a. average total costs are rising b. total costs are falling c. average variable costs are falling d. average total costs are falling