【单选题】

the major difference between pert and cpm networks is:

a. pert requires three time estimates whereas cpm requires one time estimate.
b. pert is used for construction projects whereas cpm is used for r & d.
c. pert addresses only time whereas cpm also includes cost and resource availability.
d. pert requires computer solutions whereas cpm is a manual technique.
e. pert is measured in days whereas cpm uses weeks or months.