When selecting an identifier, one should:
A. use intelligent keys.
B. use large composite keys instead of single-attribute surrogate keys.
C. choose a candidate key that will not change its value over the life of each instance of the entity type.
D. choose a candidate key such that for each instance of the entity, the attribute is guaranteed to have valid values or is null.
E. choose a candidate key that allows for duplicate values.
Answer: C