A company's target capital structure is $25$ percent debt and $75$ percent common equity. Its bonds have a $6$ percent coupon, paid annually, a current maturity of $10$ years, and sell for $$950$ The firm's marginal tax rate is $21$ percent. Which of the following is Rollins' after tax cost of debt?

$1\mathrm{.}50\%$

$6\mathrm{.}00\%$

$5\mathrm{.}29\%$

$6\mathrm{.}70\%$