Activity R on a CPM network has predecessors M and N $,$ ?and has successor S $.$ ?R has duration $5.$ ?N $\text{'}$ s late finish is $18,$ ?while M $\text{'}$ s is $20.$ ?S $\text{'}$ s late start is $14.$ ?Which of the following is definitely true?

Select one:

a $.$ ?N is a critical activity

b $.$ ?R has no slack but is not critical

c $.$ ?R is critical and has zero slack

d $.$ ?S is a critical activity.

e $.$ ?The set of times cannot all be possible.