【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.