Allison Engines Corporation has established a target capital structure of $40$ percent debt and $60$ percent common equity. The current market price of the firm's stock is $PO=$$$42\mathrm{.}7$; its last dividend was DO $=$$$5\mathrm{.}7,$ and its expected dividend growth rate is $6$ percent. What will Allison's marginal cost of retained earnings, $rs,$ be$?$

$18\mathrm{.}15\%$

$19\mathrm{.}15\%$

$20\mathrm{.}15\%$

$17\mathrm{.}15\%$

$16\mathrm{.}15\%$