Concept: The long-run average cost curve is the envelope of several short-run average cost curves.Explanation:In the long run, all factors of production are variable and a firm can choose any plant size.Each possible plant size has its own short-run average cost (SRAC) curve.The long-run average cost (LRAC) curve is drawn by touching each SRAC curve at the lowest cost point for that level of output.Thus, the LRAC curve is made up of portions of several short-run average cost curves, not marginal cost or variable cost curves.Answer: B. average cost curves