You want to make use of the object features available in the database for the form module you are developing. Which three statements about object types are true? （）

• A、An object type is a user-defined composite data type. 
• B、The attributes of an object type can be those of other object types. 
• C、A PL/SQL table of records is an example of a nested object type. 
• D、An object type must be declared as a type before the actual object itself can be declared.