Recently the corporate tax law in the U.S. changed so that firms that previously faced a marginal tax rate of close to 40% now pay tax at a flat rate of 21%. Holding everything else constant, this reduction in the tax rate faced by corporations ________.

a.) increased the after-tax cost of debt

b.) did not change the after-tax cost of debt

c.) increased the value of the deduction for interest expense

d.) decreased the after-tax cost of debt