A simple CPM network has three activities, D$,$ E$,$ and F$.$D is an immediate predecessor of E and of $F.E$ is an immediate predecessor of $F.$ The activity durations are $D=4,E=3,F=8.$

Select the correct response:

Slack at D is $3$ units

The critical path is D$-$F$,$ duration $12$

Both a and $c$ are true

The critical path is D$.$E$.$F$.$ duration $15$

Slack at $E$ is $3$ units