A firm has a cost of equity of 10 percent, a cost of preferred of 9 percent, and an aftertax cost of debt of 5 percent.

Given this, which one of the following will decrease the firm's weighted average cost of capital?

a. decreasing the debt-equity ratio

b. increasing the systematic risk level of the firm

c. redeeming the bond issue

d. issuing new equity securities

e. issuing new debt