For a Firm in Perfect Competition, a Diagram Shows Quantity

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

A) slope of the MC curve is zero.
B) MC and MR curves are parallel.
C) MC curve intersects the MR curve from below, going from left to right.
D) MC curve intersects the MR curve from above, going from left to right.