Activity R on a CPM network has a predecessor M and N, and has successor S. R has a duration 5. N's late finish is 18, while M's is 20. S's late start is 14. Which of the following is definitely true?
a. R is critical and has zero slack.
b. R has no slack but is not critical.
c. The set given times cannot all be possible.
d. N is a critical activity.
e. S is a critical activity