Why Must the Researchers Avoid Generalizing from Focus-Group Participants to the Whole

Why must the researchers avoid generalizing from focus-group participants to the whole market?

A) Participants' responses are not reliable.
B) Most of the participants are likely to be ignorant about the topic of discussion.
C) Most of the participants are likely to exhibit similar tastes and preferences.
D) The size of the group is too small and the sample is not drawn randomly.
E) The participants usually come from diverse backgrounds.