Please show reason as to which answer is correct, thank you!

Which of the following statements about the cost of capital is INCORRECT? Select one: O a. The cost of retained earnings should be zero because there is no direct cost associated with capital raised as retained earnings. O b. Weighted average cost of capital calculations should be focused on the after-tax costs. Oc. A company's target capital structure affects its weighted average cost of capital. Od. WACC should be calculated based on the market value instead of the book value. e. WACC should be calculated based on the marginal costs instead of the historical costs