Activity D on a CPM Network Has Predecessors B and C,and

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 zero slack.
E) F is critical,and has zero slack.