What is the difference between economies of scale and returns to scale? 0 a. Economies of scale are present when the expansion path is a straight line, and returns to scale are present when the expansion path is not a straight line 0 b. Economies of scale define horn.r cost changes with output, and returns to scale deline how output changes with input usage 0 c. Economies of scale are present when the long-run average cost curve is decreasing and returns to scale are present when the long-run average cost curve is increasing. 0 d. Economies of scale define how cost changes with output in the long run, and returns to scale define how cost changes with output in the short run