Suppose a firm issues bond to finance its new project it has $20$ years to maturity, a face value of $$1,000,$ and $6\%$ coupon rate. The coupons are paid semiannually. The bond currently trades at a price of $$1,050.$ Assume the corporate tax rate is $21\%,$ what is the after$-$tax cost of debt to use in the WACC calculation? $($Hint: cost of debt is an annual rate$)$

a$.4\mathrm{.}83\%$

b$.4\mathrm{.}41\%$

c$.5\mathrm{.}08\%$

d$.4\mathrm{.}55\%$