For a firm in perfect competition a diagram shows quantity on the horizontal axis and both the firm ...........

For a firm in perfect competition, a diagram shows quantity on the horizontal axis and both the firm's marginal cost (MC) and its marginal revenue (MR) on the vertical axis. The firm's profit-maximizing quantity occurs at the point where the

MC curve intersects the MR curve from above, going from left to right.

slope of the MC curve is zero.