Which of the following describes two desirable characteristics of a primary key?

A.
A primary key should be a value that may be null and may change over time.

B.
A primary key should be a value that is not null and will never change.

C.
A primary key should consist of meaningful data and a value that can be changed if needed.

D.
A primary key should not consist of meaningful data and a value that can be changed if needed.