following statement is correct about the network diagram
- The events are represented graphically by circles or nodes at the beginning and the end of activity by arrows.
-The tail end of the arrow represents the start of an activity.
-The head of the arrow represents the end of an activity.

Dummy activities often have a zero completion time and are used to represent precedence relationships that cannot be easily represented using the actual activities involved in the project.
By convention dummies are always shown as dotted arcs in network diagrams.
A dummy activity is a simulated activity of sorts, one that is of a zero duration and is created for the sole purpose of demonstrating a specific relationship and path of action on the arrow diagramming method.

The essential condition for the decompression of an activity is that An activity could be decompressed to the maximum extent of its normal time.

Dummy activities are used to Maintain the required net work.