Which Observation of Quasars Convinced Astronomers That Quasars Were Very

Which observation of quasars convinced astronomers that quasars were very distant objects?

A) extremely red spectrum, reddened by extreme interstellar absorption of the blue part of the spectrum, meaning that the source must be a very long way away
B) extreme redshift of visible Balmer and UV Lyman hydrogen emission lines, indicating high recessional velocities and hence, by the Hubble law, very large distances
C) extreme faintness at visible wavelengths; the inverse square law for visible light showed that they must be very distant
D) appearance as pointlike star images under the highest magnification, meaning that they must be very far away