Which of the following statements is CORRECT?

A$.$ If a firm's marginal tax rate is decreased, this would other things held constant, lower the cost of debt used to calculate its WACC.

B$.$ If a firm's marginal tax rate is increased, this would, other things held constant, raise the cost of debt used to calculate its WACC.

C$.$ If a firm's marginal tax rate is increased, this would other things held constant, lower the cost of debt used to calculate its WACC.

D$.$ If a firm's marginal tax rate is decreased, this would, other things held constant, have no effect on the cost of debt used to calculate its WACC.