Which one of the following statements is CORRECT?

a. A change in a companys target capital structure cannot affect its WACC.

b. The capital structure weights used in WACC calculations should be based on the market values of the individual investment capital components.

c. The capital structure weights used in WACC calculations should be based on the book values of the individual investment capital components.

d. An increase in the risk-free rate will normally lower the marginal costs of both debt and equity financing.