In entity-relationship model, the attribute type can be:(choose the best response(s))

a. multi-valued attribute

b. derived attribute

c. single valued attribute

d. simple attribute

e. multi-valued composite attribute

In relational model, the attribute type can be

a. multi-valued attribute

b. single valued attribute

c. simple attribute

d. derived attribute

e. multi-valued composite attribute

Referential integrity constraint states

a. a foreign key value must refer to an existing primary key value

b. a foreign key value can be NULL.

c. either a. or b.

d. neither a. nor b.