Neptune Corporation's bonds have 20 years to maturity with a coupon rate of 10%. Interest is paid semi-annually. The bonds sold at par value, but the firm paid flotation costs amounting to 5% of par value. The firm has a marginal tax rate of 21%. What is the firm's after-tax cost of debt for these bonds?

8.38%

4.99%

5.98%

10.61%

5.85%