Colson Corp. has planned a simple project having four tasks. The task information for the project is shown below. What is the estimated project duration? $($Enter the total number of days needed to complete the project$)$

Task Estimated Duration Immediate Predecessors

A $6$ days None

B $5$ days A

C $4$ days A

D $3$ days B$,$ C

Group of answer choices

$18$

$21$

$15$

$12$

$14$

$13$