What is a true statement in regards to creating custom report types?

What is a true statement in regards to creating custom report types?
A . The detail object in a Master-Detail relationship cannot be added as a secondary object on a custom report type.
B . Any object can be chosen unless the object is not visible to the person creating the report type through security settings.
C . When the primary object is a custom object and is deleted, then the report type and any reports created from it must be deleted manually.
D . Once a report type is saved with a standard or custom primary object, the primary object cannot be changed for that report type.

Answer: D