You are investing for your retirement. You put $40\%$ of your money into stock A$,$ with an expected return of $12\%,$ and standard deviation of $20\%.$ The rest is invested in stock B$,$ with an expected return of $10\%,$ and standard deviation of $15\%.$ The correlation coefficient between Stock A and Stock $B$ is $0\mathrm{.}5.$ What is the standard deviation of your retirement portfolio?

$17\mathrm{.}0\%$

$17\mathrm{.}5\%$

$13\mathrm{.}8\%$

$14\mathrm{.}7\%$