Which of the following statements doesNOTcorrectly explain the effect of additional debt on the weighted average cost of capital (WACC)?

a. Debtholders' prior and "fixed" claim increases the risk of stockholders' "residual" claim, so the cost of stock (r_s) goes up.

b. Additional debt also increases the pre-tax of cost of debt (r_d) because the increased risk of bankruptcy.

c. The net effect of additional debt on WACC is to increase WACC.

d. The net effect of additional debt on WACC is uncertain.