Returns to Scale Is a ______ Concept Because ______

Returns to scale is a ______ concept because ______.

A) Short-run; it's related to the law of diminishing marginal returns
B) Short-run; it deals with varying the level of one input while holding other inputs constant
C) Long-run; a firm can change its output level only in the long run
D) Long-run; it refers to changes in all of the firm's inputs