What Is Parallel Implementation

What is parallel implementation?

A) Using both the old and new systems until it is evident that the new system performs correctly
B) Discarding the old system completely and immediately start to use the new system
C) Having only a small group of people use the new system until it is evident that the new system performs correctly and then adding the remaining people to the new system
D) Implementing the new system in phases until it is evident that the new system performs correctly and then implementing the remaining phases of the new system