One Problem with Measuring Personality Characteristics of Groups of People

One problem with measuring personality characteristics of groups of people (e.g., men, women, ethnic minority groups, etc.) is that we may form stereotypes of these people. The problem with this kind of stereotype is that

A) we may not appreciate the rich variation within the groups.
B) we may prove that previous stereotypes may not apply to the groups.
C) it is conducted only on a sample of people from the groups.
D) the study may not be well controlled.