Suppose a firm has a debt$-$to$-$equity ratio $($D$/$E$)$ of $0\mathrm{.}5,$ retum on equity is $16\%$ when the firm is unleveraged, and return on debt of $12\%.$ What will its return on equity be with debts $($assume zero taxes$)?$

a$.15\mathrm{.}00\%$

b$.16\mathrm{.}67\%$

c$.20\mathrm{.}00\%$

d$.21\mathrm{.}17\%$