The WACC is computed as the weighted average of the cost of equity and the cost of debt. The firm does not have any recently issued bonds. Which is correct?

a. cost of debt is the yield on the 10 year treasury bond

b. cost of debt is the yield on the 10 year treasury bond less inflation rate.

c. cost of debt is the yield on corporate bonds with the same bond rating as the firm's debt.

d. the 91 day T-bill rate