corporate tax rate is $21\%,$ what is the after$-$tax cost of debt to use in the WACC calculation? $($Hint: cost of dobt is an annual rate$)$

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

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

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

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