Which of the following statements concerning the long - run average cost (lrac) curve is correct? a. the lrac curve is derived from a series of short-run marginal cost curves. b.the lrac curve represents the least-cost input combination of inputs for producing each level of output. c.as output increases, the amount of capital employed by the firm is held constant along the lrac curve. d.the short-run cost curve at the minimum point of the lrac curve represents the least-cost plant size for all levels of output.