Activity R on a CPM Network Has Predecessors M and N

Multiple Choice

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 22. 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 of times cannot all be possible.
D) N is a critical activity.
E) S is a critical activity.