Final answer:
In a competitive market, the marginal revenue function is flat since all firms are price takers.
Step-by-step explanation:
In a competitive market, the marginal revenue function is flat since all firms are price takers. This means that each firm in a perfectly competitive market has no control over the price of its product and can only sell its output at the prevailing market price. As a result, the marginal revenue for each additional unit sold is constant and equal to the market price.