Can someone please explain how the use of debt lowers the cost of capital when the equation for the weighted average cost of capital is:

WACC = ((Market value of common shares * cost of equity) + (market value of preferred stock * cost of preferred stock) + (market value of debt * cost of debt))/total market value

Which one of the following is a correct statement concerning the weighted average cost of capital? Taxes help lower the cost of debt and preferred stock financing. The weighted average cost of capital remains constant for a firm over time. The use of debt generally lowers a firm's cost of capital, at least initially