The _____ states that a foreign key must either match a primary key value in another relation or it must be null.

(a) entity integrity rule

(b) referential integrity constraint

(c) action assertion

(d) composite attribute

(e) None of these

If a foreign key F in relation R matches the primary key P of relation S than every value of F must either be equal to a value of P or be wholly null. The following constraint is called referential integrity constraint.