Your company currently has no debt and its WACC is $10$ percent. Your company is trying to determine its optimal capital structure, which consists only of debt and common stock. To estimate the cost of debt, the company's investment banker has produced the following table:

$\backslash$table$[[\backslash$table$[[$Percent Financed with$],[$Debt$]],$Rating,$\backslash$table$[[$Before$-$Tax Cost of$],[$Debt$]]],[10\%,$AA$,7\mathrm{.}0\%$