Assumption Based CR

AnuragRatna wrote:Spectroscopic analysis has revealed the existence of frozen nitrogen, methane, and carbon monoxide on the surface of Pluto. Such ices have a tendency to vaporize, producing an atmosphere. Since the proportion of any gas in such an atmosphere depends directly on how readily the corresponding ice vaporizes, astronomers have concluded that the components of Pluto’s atmosphere are nitrogen, carbon monoxide, and methane, in order of decreasing abundance.

The astronomers' argument relies on which one of the following assumptions?

(A) There is no more frozen nitrogen on the surface of Pluto than there is either frozen carbon monoxide or methane.
(B) Until space probes reach Pluto, direct analysis of the atmosphere is impossible.
(C) There is no frozen substance on the surface of Pluto that vaporizes more readily than methane but less readily than carbon monoxide.
(D) Nitrogen is found in the atmosphere of a planet only if nitrogen ice is found on the surface of that planet.
(E) A mixture of nitrogen, carbon monoxide, and methane is characteristic of the substances from which the Solar System formed.

Conclusion: The components of Pluto's atmosphere are nitrogen, carbon monoxide, and methane, in order of decreasing abundance.

The assumption is WHAT MUST BE TRUE for the conclusion to be valid.
Apply the NEGATION test.
Since the assumption is WHAT MUST BE TRUE, when the correct answer choice is negated, the conclusion will be invalidated.

Answer choice C, negated:
On the surface of Pluto are frozen substances A through Z that vaporize more readily than methane but less readily than carbon monoxide.
The negation of C implies that the components of Pluto's atmosphere will be nitrogen, carbon dioxide, substances A through Z, and methane, invalidating the conclusion that the components of Pluto's atmosphere are nitrogen, carbon monoxide, and methane, in order of decreasing abundance.
Since the negation of C invalidates the conclusion, C is WHAT MUST BE TRUE for the conclusion to be valid.

The correct answer is C.

Some test-takers will be attracted to A.
Answer choice A, negated:
There is more frozen nitrogen on the surface of Pluto than there is either frozen carbon monoxide or methane.
The negation of A SUPPORTS the conclusion that nitrogen will be the most abundant component of Pluto's atmosphere.
Since the negation of A does not invalidate the conclusion, eliminate A.

@GMATGuruNY Thanks for your detailed & crystallize explanataion.

@GmatGuruNY: Thank You for your reply

Mitch did a great job breaking this one down-- just wanted to add some extra info to help you deal with Assumption questions!

An assumption is an unstated part of the argument on which the argument depends. Remember our equation:

Evidence + Assumption = Conclusion

For example, consider this argument:

American Idol holds a televised talent contest for all eligible singers. Monica has a beautiful soprano voice, so she will likely make it onto the show.

The conclusion is that Monica will make it onto the show based on the evidence that American Idol
showcases eligible singers. It must be true, therefore, that Monica is eligible, otherwise she wouldn’t be “likely” to make it on the show.

This type of assumption is a necessary assumption, because it MUST be true in order for the conclusion to be true. However, it is not a sufficient assumption because it doesn’t sufficiently prove the conclusion is true (Monica will make it onto the show). Thought it’s necessary that Monica is eligible, just because she IS eligible doesn’t mean she will make it onto the show. She could be eligible and not make it onto the show.

These type of assumptions usually have the words “depends” or “requires” in the question-stem. Some common question-stems:

• “Which of the following is an assumption on which argument depends?”
• “The conclusion drawn requires the assumption…”
• “The argument relies on which of the following assumptions..”
• “The conclusion above depends on which one of the following assumptions…”

Remember that in order for there to be a necessary assumption, the argument can’t be a strong argument!

Always read the passage in an Assumption question with a critical eye. The argument must be flawed in order to require an assumption to make it stronger! The flaw in a “necessary assumption” question is usually related to the new information in the conclusion.

Assumption Strategy

Step 1 – Identify the Type of Assumption (Necessary or Sufficient?)
Step 2 – Break Down the Argument (find the conclusion and evidence)
Step 3 – Write Down a Prediction
Step 4 – Eliminate Non-Assumptions
Step 5 – Select the Choice Most Required by the Conclusion

Good luck rockin' those Assumptions!