Why Is It Thought That Quasars Probably Spend Only a Fairly

Multiple Choice

Why is it thought that quasars probably spend only a fairly short time in their highlyluminous phase?

A) because there aren't many quasars around today
B) because there aren't any quasars closer than about a redshift of 1
C) because all quasars are at about the same epoch, narrowly defined in time
D) because a black hole could not power a highly luminous quasar for more than a few millionyears
E) because quasars are known to be the consumers of massive galaxies