Which statement is NOT CORRECT?

		A foreign key can be NULL.
		A superkey is a key that cannot have any of its attribute types removed. This is also called a minimal key.
		In mapping an EER N:M relationship type to the relational model, we create a new relation R. Its primary key is the combination of the foreign keys referring to the primary keys of the relations corresponding to the participating entity types.
		A relation is in 3NF if it is in 2NF and no non-prime attribute type of R is transitively dependent on the primary key.