The after$-$tax cost of debt for a firm which has a marginal tax rate of $40\%,$ is $6\%.$

Calculate the before$-$tax cost of debt.

a$.$

$8\mathrm{.}1\%$

b$.$

$10\mathrm{.}0\%$

c$.$

$6\mathrm{.}0\%$

d$.$

$9\mathrm{.}2\%$