What's the appropriate tax rate to be used in WACC?

A. The simple average of the tax rates that would have been paid on the taxable income shielded by the interest deduction

B. The weighted average of the marginal tax rates that would have been paid on the taxable income shielded by the interest deduction

C. The weighted average of the firm's discounted current marginal tax rates

D. The highest applicable tax rate charged on the firm's income