Why Is the Energy Source for Active Nuclei Like Seyferts

Multiple Choice

Why is the energy source for active nuclei like Seyferts thought to be compact?

A) The sources appear to be single stars in photos.
B) Their energy appears to be non-stellar synchrotron radiation.
C) The light can vary over short time intervals.
D) We know the masses of the black holes that lurk there, and can find their radii.
E) They are all strong radio sources, with assigned frequencies by their sizes.