$3.$ A primary key in a table $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.$a$.$ must be different from a primary key in another tableb. must be different from foreign keys in another tablesc. can be the same value as other primary key values in the same tabled. must not contain NULL$4.$ Referential integrity is where a$.$ references in the database are identical to references in an ER diagram. b$.$ foreign key always refers to a primary key value or is NULL. c$.$ primary keys in one table always refer to primary keys in another table. d$.$ foreign keys in one table always refer to foreign keys in another table.