What was the risk $($standard deviation$)$ of a portfolio comprised $50\%$ of the S&P$500$ and $50\%$ of the NASDAQ over the period specified? $($Pick the closest answer$)$

Question $5$ options:

$18\%$

$17\mathrm{.}5\%$

$17\%$

$16\mathrm{.}5\%$