Which of the following statements is most correct?

a. An increase in the risk-free rate will have no impact on the marginal costs of both debt and equity capital financing.

b. The weighted average cost of capital for a given capital budget level is a weighted average of the marginal cost of each relevant capital component that makes up the firm's target capital structure.

c. The weighted average cost of capital is calculated on a before-tax basis.

d. Statements a and c are correct.

e. All of the statements above are correct.