For an activity in a CPM analysis the Early Start time is $15$ and the Late Start time is $20.$ Which of the following statements is true?

Question $8$ options:

a$)$

The activity is not on the critical path.

b$)$

The duration of this task is $20.$

c$)$

The slack for this activity is $20.$

d$)$

The activity's late start must happen before its early start.