How Has the Number of Quasars Varied Over the History

Multiple Choice

How has the number of quasars varied over the history of the universe?

A) The number of quasars has remained approximately constant over time with new quasars being formed to replace those which deplete their fuel and die out.
B) Quasars were much more numerous 11 or 12 billion years ago, but since then the supermassive black holes in their centers have depleted their available gas and dust, their accretion disks have disappeared, and they are no longer quasars.
C) It takes billions of years to accumulate enough material to form a supermassive black hole, the heart of a quasar. Thus, the number of quasars has grown steadily as more and more supermassive black holes are formed.
D) The number of quasars has decreased steadily as they have evolved into other types of AGN like radio galaxies and BL Lacertae objects.