Which one is right option from the following - 

1. Monte Carlo analysis is a very effective maunal analysis.

2. Crtical path can include a dummy activity.

3. Only one crtical path can exist in any project.

4.Fast tracking is better than crashing.

By looking choice and using elimination technique, i got the right anser - which is 2. However, can someone help me understanding what is dummy activity means here. 

Thanks,

Trish