Which one of the following statements is CORRECT? O a. The capital structure weights used in WACC calculations should be based on the book values of the individual investment capital components. O b. An increase in the risk-free rate will normally lower the marginal costs of both debt and equity financing. Oc. A change in a company's target capital structure cannot affect its WACC. d. The capital structure weights used in WACC calculations should be based on the market values of the individual investment capital components