For a firm, what does "constant returns to scale" refer to?

Select one:

a. Short-run average total cost does not change as the quantity of output changes.

b. Short-run average total cost changes at a constant rate as the quantity of output changes.

c. Long-run average total cost changes at a constant rate as the quantity of output changes.

d. Long-run average total cost does not change as the quantity of output changes.