The Error Message Index or Primary Key Cannot Contain a Null

Multiple Choice

The error message Index or primary key cannot contain a Null Value. will be generated by attempting which of the following?

A) A user tries to enter a record in the table Countries that doesn't contain data in the primary key.
B) A user tries to add a record to a table that is related to the Countries table, and that record contains a foreign key value that does NOT correspond to a primary key value in the Countries table.
C) A user tries to enter a record in the table Countries with the same primary key as another record.
D) A user tries to request a joined table using LINQ.