Maximizing the Profit? Margin? According to the marginal? principle, the firm should choose the quantity of output at which price equals marginal cost. A tempting alternative is to maximize the? firm's profit? margin, defined as the difference between price and? short-run average total cost. Using this? approach, which of the following would best describe the? firm's short-run supply? curve? Assume the firm will shut down rather than operate at a loss. A. It is equal to the average cost? curve, but shifted up. B. It is equal to the average cost curve. C. A vertical line at the quantity that minimizes average cost. D. A vertical line at the quantity that minimizes average cost for prices above minimum average cost.