For any given output level, a firm's long-run costs

a. are always greater than or equal to its short-run costs.
b. are usually greater than or equal to its short-run costs except in the case of diminishing returns to scale.
c. are always less than or equal to its short-run costs.
d. are usually less than or equal to its short-run costs except in the case of diminishing returns to scale.

c