Activity 7-8 is directly preceded by two activities—activity 5-7 and activity 6-7. Activity 5-7 has an expected time of five and an ES time of six. Activity 6-7 has an expected time of seven and an ES time of seven. Activity 7-8 has an expected time of 10. The ES for activity 7-8 is:
A. 11
B. 14
C. 21
D. 24
E. 25