The estimated mean time for an activity with a most likely time $($m$)$ equal to $12$ weeks, an optimistic time $($a$)$ equal to $9$ weeks, and a pessimistic time $($b$)$ equal to $18$ weeks is

$12\mathrm{.}75$ weeks.

$9\mathrm{.}0$ weeks.

$13\mathrm{.}0$ weeks.

$12\mathrm{.}5$ weeks.