What is meant by the cost—time option?

1. When employing PERT/COST, the cost—time option applies. What is meant by the cost—time option? How can it affect the critical path?
2. What is the purpose of a WBS? What is the difference between a WBS, an SWBS, and a CWBS? How do work packages relate to the WBS? Construct a WBS for a program of your choice.