Consider a five$-$task process, with five manual tasks to be performed before the product is completed, as diagrammed below. The task times, shown in minutes, are the times that it takes one worker to perform that task for one unit of product. This line is staffed with six workers, two of whom are assigned to workstation D$.$ Assume that each of the tasks $($tasks A through E$)$ are never split among multiple workers. Thus, the two workers doing task D work on individual units, each doing all of task D on the particular unit on which he or she is working. Assume that there is ample equipment so that all workers can perform their tasks. Which task$/$station is the bottleneck of this line?

$1$ worker

$1$ worker

A

$6$ mins.

$9$ mins.

B

$1$ worker

C

$1$ worker

D

$1$ worker

$15$ mins.

E

$12$ mins.

$1$ worker

$9$ mins.

D

$15$ mins.