Why Isn't Listening to Callers on a Talk Radio Station

Why isn't listening to callers on a talk radio station a good way of measuring public opinion?

A) They are a self-selecting group who may not hold views that are typical of most Americans.
B) They are generally wealthier than the average American, with the time to call in their opinions, thus they don't represent a typical American.
C) Callers to a station are always a self-selecting group of conservatives, so they don't represent the average American.
D) Talk radio is heard by only local stations, so it can't represent the country.
E) Most people don't listen to radio as much as they watch television, so the audience is not representative of the average American.