Why Do Astronomers Think That There Are Fewer Quasars Today

Why do astronomers think that there are fewer quasars today than there were billions of years ago?

A) quasars are seen when the supermassive black hole at the center of a galaxy has a lot of "food to eat" (stars and gas) and the amount of available food tends to decrease with time
B) quasars are supermassive black holes at the centers of galaxies, and such black holes are constantly losing mass and thus getting less and less able to pull on anything
C) quasars tend to merge all the time, and so there are fewer and fewer of them as time goes on
D) quasars are caused by many supernovae going off at the same time, and massive stars only exploded in the early history of the universe; they don't explode today
E) no one has the slightest idea on how to answer this question; it is completely unsolved