Which of the following statements about the cost of capital is INCORRECT? Select one: a. A company's target capital structure affects its weighted average cost of capital. O b. WACC should be calculated based on the marginal costs instead of the historical costs. c. WACC should be calculated based on the book value. O d. Weighted average cost of capital calculations should be focused on the after-tax costs. e. The reason we must assign a cost of capital to retained earnings involves the opportunity cost principle