What Is a Major Limitation of "Nationally Representative" Survey Data

What is a major limitation of "nationally representative" survey data?

A) Minority groups are typically underrepresented.
B) When minority groups are included, it is often from groups that are experiencing significant problems.
C) In order to appear representative, researchers often over-include people from minority groups resulting in skewed data.
D) Most people in minority groups refuse to participate in research studies.