What Observation in Astronomy, Made AFTER the Discovery of Quasars

What observation in astronomy, made AFTER the discovery of quasars, was a big help to astronomers in figuring out what quasars really were?

A) the understanding that most stars become white dwarfs when they die and white dwarfs are much smaller than main-sequence stars
B) the discovery that the Milky Way Galaxy has a black hole at the center with enough mass for 4 million Suns
C) the discovery that the galaxies mostly show a red shift
D) the discovery of exoplanets (planets orbiting other stars) throughout the Galaxy
E) the discovery of Herbig-Haro objects around newly forming stars