Which One of the Following Conditions Violates the Assumptions of PERT/CPM

Multiple Choice

Which one of the following conditions violates the assumptions of PERT/CPM networks?

A) Two activities tied together by an arc are overlapping and can be worked on simultaneously.
B) Costs increase linearly as activity time is reduced below its normal time.
C) There can be more than one critical path in a network.
D) Some activities can have zero variance.