$23.$ Topstone Industries has a cost of equity of $10\%$ and a cost of debt of $6\%.$ Topstone's debt$-$to$-$equity ratio is $0\mathrm{.}5.$ The corporate tax rate is $50\%.$ What is the appropriate discount rate to be used under the APV method to value Topstone?

A$.$

$9\mathrm{.}8\%$

B$.$

$7\mathrm{.}7\%$

C$.$

$10\mathrm{.}0\%$

D$.$

$9\mathrm{.}2\%$

E$.$

$10\mathrm{.}8\%$