Which of the following is a correct formula for calulating the cost of capital? Select one: a. WACC = weighted cost of debt + weighted cost of preferred stock + weighted cost of common stock b. WACC = (after-tax cost of debt + cost of preferred stock + cost of common stock )/3 c. WACC = weighted after-tax cost of debt + weighted cost of preferred stock + weighted cost of common stock d. WACC = weighted after-tax cost of debt + weighted after-tax cost of preferred stock + weighted after-tax cost of common stock