Which of the following mathematical expressions is used to calculate the after$-$tax cost of debt, rdT$?$

a$.$ rdT $=$ Firm's internal rate of return $$ Dividend payments

b$.$ rdT $=$ Firm's internal rate of return $$ Tax payments

c$.$ rdT $=$ Bondholders' required rate of return $($YTM$)$ Tax savings

d$.$ rdT $=$ Firm's internal rate of return $$ Tax savings

e$.$ rdT $=$ Bondholders' required rate of return $($YTM$)$ Interest payments