1 (System)

This is a place where we defined what the namespaces going to be consumed in the program/application are. Generally, this is called using a statement, which includes the usage of external, internal, or any other namespaces.

System is a typical namespace that contains a lot of fundamental classes. For more information, refer to https://docs.microsoft.com/en-us/dotnet/api/system?view=netcore-2.0.