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?

Select one:

a. D is critical, and has zero slack.

b. C is completed before B.

c. D has no slack but is not critical.

d. F is critical, and has zero slack.

e. B is a critical activity.