Consider the following Activity$-$On$-$Node $($AON$)$ project $($activity completion times are in days$).$

it is also a project network showing where node A is starting node and node F is the destination node with different network layout. The numbers above each node indicate the duration time to finish the each node.

Refer to the figure. What is the slack time, in days, associated with activity D$?$

A$.$

$1$

B$.$

$4$

C$.$

$3$

D$.$

$0$

E$.$

$2$