Assuming no fixed costs are avoidable in the short​ run, a perfectly competitive​ firm's short-run supply curve is A. the portion of its average variable cost curve that lies below its average total cost curve. B. the portion of its average variable cost curve that lies above its average total cost curve. C. the portion of its marginal cost curve that lies above its average variable cost curve. D. the portion of its marginal cost curve that lies above its average total cost curve.