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Here are two different thought process of mine for this question:

1.

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

In decreasing abundance - Nitrogen, Carbon Monoxide, and Methane

So if there is no frozen substance on the surface of pluto that vaporizes more readily than methane that means methane is the most readily substance that can be vaporized

so in order of being vaporized it goes - Methane, Carbon Monoxide, Nitrogen

My problem with C is that says "but less readily than carbon monoxide"

what about nitrogen? doesn't nitrogen vaporize less readily than carbon monoxide?

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I have a strong feeling I'm missing something very important.

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2.

Nitrogen - Carbon Monoxide - Methane

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

XYZ = unknown substance

Nitrogen - Carbon Monoxide - Methane

so Nitrogen - (XYZ) - Carbon Monoxide - Methane (XYZ)

is this the solution? Is this answer telling me that I need to assume that (XYZ) cannot be in these two places at once?

The scientists are saying that the atmosphere is made up of nitrogen, carbon monoxide and methane, in order of decreasing abundance. That means that nitrogen is the most-readily vaporized, followed by carbon monoxide, then finally methane. You have the order of vaporization backwards in your post.

Because the composition of the atmosphere depends directly on how easily the frozen surface material vaporizes, for the scientists to be correct about the composition of the atmosphere, then (C) must be correct; if there were any frozen chemical on the surface that vaporized more readily than methane but less readily than carbon monoxide, then it would have to make up part of the atmosphere. Specifically, it would be more abundant in the atmosphere than methane, but less abundant than carbon monoxide.

In short, if (C) **isn't** true, than the scientists' claim about the atmosphere would be wrong, because that means they would have left out a component of the atmosphere.

I'm just trying to understand the nature what answer choice (C) is trying to do.

Is (C) an attempt to rule out the idea of other substances existing on Pluto?

ScottRiqui wrote: In short, if (C) **isn't** true, than the scientists' claim about the atmosphere would be wrong, because that means they would have left out a component of the atmosphere.

okay, I understand