Activity R on a CPM Network Has Predecessors M and N,and

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 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.