What Makes Astronomers Believe That the Energy Source in Quasars

What makes astronomers believe that the energy source in quasars is only a few light months across (the distance light travels in a few months) ?

A) we have only seen the light from such quasars for a few months so far
B) a few light months is about the size of a typical galaxy, and astronomers think quasars are active galaxies
C) quasars show variations in their energy output that have a period of a few months
D) 10 light months is about the size of the belt of comets around the solar system, and that is where we believe quasars are located
E) the Hubble Law requires objects with high red shifts (like quasars) to be pretty close to us