Although a firm's existing mix of financing sources may reflect its target capital structure, it is ultimately OA. the risk- free rate of return that is relevant for evaluating the firm's future investment opportunities OB, the risk-free rate of return that is relevant for evaluating the firm's future financing opportunities OC. the marginal cost of capital that is relevant for evaluating the firm's future investment opportunities OD the internal rate of return that is relevant for evaluating the firm's future investment opportunities