An electronic store uses the order$-$up$-$to model to manage its inventory of Wireless Headphones, aiming to maintain a $90\%$ service level. The daily demand follows a normal distribution with a mean of $35$ units$/$day and a standard deviation of $6$ units$/$day$.$ The effective lead time is $5$ days. How much safety stock should the store hold to maintain its intended service level?

$27$ units

$13$ units

$12$ units

$17$ units