A simple CPM network has three activities, D, E, and F. D is an immediate predecessor of E and of F. E is an immediate predecessor of F. The activity durations are D=4, E=3, F=8. Select one: a. The critical path is D-F, duration 12. b. both the critical path is D-F, duration 12 and slack at D is 3 periods. c. Slack at E is 3 periods. d. the critical path is D-E-F, duration 15. e. Slack at D in 3 periods.