Why might learning by doing decrease the long-run average cost of production? a. as output increases, the production process begins to exhibit decreasing returns to scale. b. as output increases, firms begin to experience economies of scale. c. as cumulative output increases, engineers gain experiences that decrease costs with increasing defects. d. as cumulative output increases, workers become more adept, and the speed at which they produce increases. e. as cumulative output increases, managers become unable to schedule the production process as efficiently.