Which of the following statements about the cost of capital is INCORRECT?

a. The reason we must assign a cost of capital to retained earnings involves the opportunity cost principle.

b. A company's target capital affects its weighted average cost of capital. (I know B is not the answer)

c. Weighted average cost of capital calculations should be focused on the before-tax costs.

d. WACC should be calculated based on the marginal costs instead of the historical costs.

e. WACC should be calculated based on the market value instead of the book value.

Note: As said above, I thought the answer was B but I was wrong.