What happens whan a domain is created in AD

HI All,

Just need an clarification from below mentioned question, I am little confused about the answer:

what happens when a "Domain" is created in active directory:

weather:

1. A new top level tree is created in the directory.

2. An OU is created and provisioned to contain  "CN"=(common name/user) Object.

3. An unique entity is defined in active directory forest for directory objects.

4.  the schema of the directory is expanded to include the new domain branch.

Please help to answer which is appropriate of this.

Thanks & Regards