Which of the following statements is most correct?

a. A firm can use retained earnings without paying a flotation cost. Therefore, while the cost of retained earnings is not zero, the cost of retained earnings is generally lower than the after-tax cost of debt financing.

b. If a firm finds that the cost of debt financing is currently less than the cost of equity financing, an increase in its debt ratio will always reduce its weighted average cost of capital.

c. The capital structure that minimizes the firm's weighted average cost of capital is also the capital structure that maximizes the firm's stock price.

d. The capital structure that minimizes the firm's weighted average cost of capital is also the capital structure that maximizes the firm's earnings per share.