Activity R on a CPM network has predecessors M and N, and has successor S. R has 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) S is a critical activity
b) The set of given times cannot all be possible
c) R has no slack but is not critical
d) N is a critical activity
e) R is critical and has zero slack