Activity D on a CPM Network Has Predecessors B and C

Multiple Choice

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) B is a critical activity.
B) C is completed before B.
C) D has no slack but is not critical.
D) D is critical, and has a slack of two days.
E) F is critical, and has zero slack.