The before$-$tax cost of debt for a firm, which has a $45\%$ marginal tax rate, is correctly calculated at $11\%.$

Calculate the after$-$tax cost of debt.

a$.$

$6\mathrm{.}05\%$

b$.$

$4\mathrm{.}20\%$

c$.$

$12\mathrm{.}06\%$

d$.$

$9\mathrm{.}60\%$