$(16-14)$ WACC and Optimal Capital Structure

F$.$ Pierce Products Inc. is considering changing its capital structure. F$.$ Pierce currently has no debt and no preferred stock, but it would like to add some debt to take advantage of the tax shield. Its investment banker has indicated that the pre$-$tax cost of debt under various possible capital structures would be as follows:

$\backslash$table$[[\backslash$table$[[$Market Debt$-],[$to$-$Value$],[$Ratio$],[(w_d)$