Which of the following statements is true about primary keys?

a$.$ The function of the primary key is to guarantee entity integrity, not to "describe" the entity.

b$.$ The function of the primary key is to guarantee entity integrity, and to "describe" the entity.

c$.$ A primary key is the attribute or combination of attributes that uniquely identifies entity instances in an entity set.

d$.$ A primary key is the attribute or combination of attributes that identifies entity instances in an entity set.

e$.$ A and C

f$.$ A and D

g$.$ B and D