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Activity R on a CPM network has predecessors M and N$,$ and has successor S$.$ R has duration $5.$ N$\text{'}$s late finish is $18,$ while M$\text{'}$s is $20.$ S$\text{'}$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$)S$ is a critical activity.

E$)N$ is a critical activity.