Quasars—celestial objects so far away that their light takes at least 500 million years to reach Earth—have been seen...

Ravi on March 9, 2019

@Shiyi-Zhang,

Happy to help. You're wanting to know why (D) is incorrect. Let's take
a look at the stimulus first.

The stimulus tells us that quasars produce more light than 90 billion
suns. We're also told that quasars have been seen on earth since 1963.
We know that nothing that burns at a rate that produces as much light
as a quasar can exist longer than 100 million years. We also know that
quasars' light takes at least 500 million years to reach earth.

The question asks, "If the statements above are true, which one of the
following must also be true on the basis of them?"

We know that the quasars' light takes at least 500 million years to
reach earth. We also know that quasars burn out in 100 million years.
Together, what does this mean? Well, the quasars that we see on earth
must no longer exist! This is the inference we can push out from the
premises.

(D) says, "Nothing that is as far from Earth as quasars are can
continue to exist for more than about 100 million years."

The problem with (D) is that we know that quasars only last for up to
100 million years because of how brightly they burn. It has nothing to
do with their distance from the earth. There could easily be another
type of celestial body that doesn't burn that brightly but lasts for a
billion years. Thus, we can't infer this.

(E) says, "No quasar that has ever been seen from Earth exists any longer."

(E) must be true because we know that that the closest quasars are 500
million light years from the earth. Since we know that they only last
for 100 million years, the quasars people see from earth must no
longer exist. (E) is what we anticipated and is the correct answer.

Does this make sense? Let us know if you have any questions!