I am between C and D.

(A) There is no more frozen nitrogen on the surface of Pluto than there is either frozen carbon monoxide or methane.
- Out. Does not tells us there is more frozen nitrogen than frozen carbon monoxide/methane


(B) Until space probes reach Pluto, direct analysis of the atmosphere is impossible.
- Out of scope

(C) There is no frozen substance on the surface of Pluto that vaporizes more readily than methane but less readily than carbon monoxide.
- This one explains why there is a decreasing abundance from nitrogen to carbon monxide to methane.

(D) Nitrogen is found in the atmosphere of a planet only if nitrogen ice is found on the surface of that planet.
- Does not explain the abundance. We're already told the ice vaporize to produce an atmosphere, so this choice does nothign more than to give a redundant premise

(E) A mixture of nitrogen, carbon monoxide, and methane is characteristic of the substances from which the Solar System formed.
- out of scope

I'll go with C

(D)

I agree with C too.

makes sense.. note to self.. don't second guess thanks ywilfred

The main problem with this is the "only if". The scientists do not have to make this assumption. They could use this assumption instead: Nitrogen will be found in the atmosphere (B) if nitrogen ice is found on the surface of the planet (A). They already know that the ice is found, it is sufficient for them to conclude that nitrogen can be found in the atmosphere based on the assumption that A is sufficient for B. They don't need to assume A is necessary for B. There might be other cases where there are nitrogen in the air without nitrogen ice in the ground.

Here's a diagramming of the passage I did.

1) Spectroscopic analysis revealed existence of frozen nitrogen, methane and carbon monoxide on surface of pluto
2) Such ice have tendency to vaporize, producing an atmosphere
3) Proportion of gas in atmosphere depends directly on how readily corresponding ice vaporize
4)Astronomers concluded that components of Pluto's atmosphere are nitrogen, Carbon monixide and methane in decreasing abundance.

1, 2 and 3 are premises --> All facts
4 is a conclusion --> A claim (made by the astronomers)

In decreasing abundance means in Pluto's atmosphere, there is more nitrogen, than carbon monxide, and more carbon monxide than methane. It's talking about the gases that make up the atmosphere, and telling us which gas is higher in proportion.

Based on premise 3, if the conclusion is true, nitrogen vaporizes the fastest, followed by carbon monoxide, and methane.

Now we'll look at answer choice (C).

"There is no frozen substance on the surface of Pluto that vaporizes more readily than methane but less readily than carbon monoxide. "

In order for the astronomer's claim that nitrogen is highest in proportion followed by carbon monoxide then methane, we need a statement that tells us that the proportion is indeed in this order.

(C) does that perfectly. It tells us there is no frozen substance (e.g. frozen oxygen) that vaporizes faster than methane but slower tan carbon monoxide

C doesn't contradict the passage.

We're told nothing on pluto vaporizes faster than methane, and slower than carbon monoxide.

That means there cannot be anything in between methane and carbon monoxide. The order Nitrogen->Carbon Monixide->Methane will not be changed.

In fact, A can be refuted to say there are other substances like frozen oxygen, and they could be higher in quantity. C cannot be challenged by this assumption even if you really had frozen oxygen, because we know that this frozen oxygen is not going to vaporize faster than methan and slower than carbon monoxide.

Hi Chunjuwu, let me give an example illustrating what I meant. We know that the three gases, nitrogen, carbon monoxide and methane must go in this order from most to least: Nitrogen -> Carbon Monoxide -> Methane.

Now if there's another substance, say frozen oxygen that vaporiese faster than methan but slower than Carbon Monoxide, we will get:
Nitrogen -> Carbon Monoxide -> Oxygen -> Methan. This is exactly the situation C helps us to avoid getting into. It says no other substance, vaporizes faster than Methane, and slower than Carbon monoxide. This means we will never end up in this situation:

Nitrogen -> Carbon Monoxide -> Other Subsance -> Methane.

And if we can never get into the situation above, the conclusion will hold.

Hope what I explained is clear for you. Let me know if you need more

You're most welcome