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. Which of the following is true Select one: O a. D) Slack at Eis 3 periods. O b. A) The critical path is D-E-F, duration 15. O c. B) The critical path is D-F, duration 12. O d. C) Slack at Dis 3 periods