Attached table shows all activities to finish a project. The crashing cost is in thousand dollar. Activity Immediate Predecessor Normal Time (weeks) Crashing Cost (1st week) Crashing Cost (2nd week) A - 6 8 - B A 4 5 6 C A 4 - - D B 5 6 6 E C 6 5 6 F D, E 6 7 10 Which activity has non-zero slack time? B and C B and D C and E only B