Which of the following is wrong?

Select one:

a. Economies of scale refer to the production of two or more goods and occur when joint production is less costly than the sum of the costs of producing each good separately.

b. When the firm experiences economies of scale, its long-run average cost curve is downward sloping.

c. The expansion path describes the cost-minimizing combination of inputs that the firm chooses for every output level.

d.

When costs increase proportionately with output, the firm's long-run average cost curve is horizontal.