If a firm is maximizing its profits while earning normal profit, which of the following must be true?

a. Marginal revenue, total cost, and marginal cost are all equal.

b. Marginal revenue equals average total cost, and total revenue is maximized.

c. Marginal revenue equals marginal cost, and total cost equals total revenue.

d. Marginal revenue is greater than average total cost, and marginal cost is at its minimum.

e. Marginal revenue equals average total cost, and marginal cost is at its minimum.