Activity D on a CPM network has predecessors B and C. and has successor F. D has duration 6. B’s earliest finish is 18. While C’s is 20. F’s late start is 26. Which of the following is definitely true

A. D is critical and has zero slack

B. B is a critical activity.

C. F is critical, and has zero slack.

D. D has no slack but is not critical.

E. C is completed before B.