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)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.