A PERT$/$CPM activity has an optimistic time estimate of $4$ days, a most likely time estimate of $6$ days, and a pessimistic time estimate of $10$ days. The standard deviation of this activity is

$\frac{7}{9}$

$\frac{7}{6}$

$\frac{1}{3}$

$\frac{2}{3}$

$1\mathrm{.}0$

None of the above