Activity D on a CPM network has predecessors B and C, and has successor F. D has duration 5. B's earliest finish is 15 , while C's is 18 . F's late start is 23 . Which of the following is definitely TRUE? C must be started after B is finished. B has no slack, and is critical. D is critical, and has a slack of five days. C has no slack, and is critical. F is not critical, and has slack