Why Do Psychologists Periodically Revise a Test Such as the MMPI

Why do psychologists periodically revise a test such as the MMPI or the WAIS-III?

A) Because of changes in society, it has become necessary to make the tests easier and easier.
B) After the test has been used for a few years, many people have heard what is on the test before they take it.
C) The psychologists who administer the tests become bored with using exactly the same test year after year.
D) The norms that were appropriate in the past have become obsolete.