1) The Spotted Mole is a rodent that burrows underground and eats all forms of vegetable matter.   Farmers are concerned that this mole could eat some of their commercial fruits, planted in above-ground planters and bins.   The farmers need not worry about the Mole, though, because throughout the region in which the Spotted Mole is found, birds of prey such as hawks and falcons are active, and these birds would prey upon the Spotted Mole if the mole came above ground at all.  Therefore, the Spotted Mole poses no threat to these totally above-ground fruits.

Which one of the following is an assumption on which the argument depends?

(A) The birds of prey capture and kill every single Spotted Mole that comes above ground.

(B) Some land-based mammals active in this region, such as fox, will also hunt and eat the Spotted Mole on a regular basis.

(C) No other animal could pose as significant a threat to the above-ground fruits as could the Spotted Mole.

(D) The times of day the Spotted Mole feeds are the same as the times of day that the birds of prey are in the air.

(E) Larger burrowing mammals, such as badgers or weasels, can dig up the burrows of the Spotted Mole, endangering those that remain underground.

An explanation will follow this article.

 

Finding the assumption

In the next section, I will talk about the Negation Test for verifying that a statement is an assumption of the argument, but first I want to dispel any idea that the Negation Test is the only way to find an assumption.   There's a much more basic approach, which I will call the "Bridge" Approach to finding assumption.  Suppose we had the following nonsense CR argument:

2) Blah blah blah blah blah blah both P and Q.  Blah blah blah P more than Q blah blah.   Blah blah blah Q instead of P blah blah.  Therfore, blah blah T.

Which one of the following is an assumption on which the argument depends? 

A. Blah blah blah P or Q blah blah

B. Blah blah blab Q instead of P

C. Blah blah P and Q blah blah T

D. Blah blah blah W and T

E. Blah blah blah without any T

All the premises of the argument were discussing P & Q, and then suddenly the conclusion leaps to something completely different, T.  Clearly, the assumption of the argument must be something that links, that creates a bridge between (P + Q) and T.   Without knowing any of the details of this argument or even what P & Q & T even are, it seems likely that (C) is the answer to the nonsense question, because it is the only one that simultaneously mentions both P + Q and T.

This was a little extreme, but many GMAT CR arguments have premises discussing one topic, and then they make a significant leap to a related but different topic, and of course, the assumption is what bridges the premise-topics and the conclusion-topic.   This does not work for all CR assumption questions, but when it does work, it is a lightning fast approach to these questions.

 

The Negation Test

This test always works, and it always a good way to verify an assumption.   Here's the rule:

If you negate a statement, and it's still possible to imagine that the conclusion is still true even with this negated statement, then that original statement is definitely not an assumption of the argument.  

If you negate a statement, and this negated statement is a devastating objection which shatters the argument and makes the conclusion untenable, then that original statement is an assumption of the argument.  

Here's a super-simple argument, with only three answer choices:

3) Alex likes this movie.  Therefore, Betty will like it. 

Find the assumption of the argument

A. Both Alex & Betty liked the same movie last year

B. Carla didn't like this movie, and last year, she & Betty liked the same movie.

C. Betty likes the movies that Alex likes.

This is supposed to be a very easy question.  We want the assumption, so let's just apply the Negation Test to all three answers:

The negation of (A): Alex & Betty didn't like the same movie last year —- a vaguely weakening objection, but that was last year, and this is this year: what Betty likes now might be the same as what Alex likes now.  It's at least conceivable that the negation of (A) could be true and yet the conclusion to this argument would hold.   Therefore, (A) is not an assumption of the argument.

The negation of (B): Carla liked this movie, and last year, she & Betty liked the same movie. OR Carla didn't like this movie, and last year, she & Betty didn't liked the same movie.  —- Either way, this answer just introduces the variable of another person.  We don't know how fickle Carla is, and whether her taste overlaps in any meaningful or consistent way with Betty's.  Therefore, it is possible that either of these negations could be true and the conclusion would still hold.   Therefore, (B) is not an assumption of the argument.

The negation of (C): Betty doesn't like the movies that Alex likes. —- Hmm, if this is true, and if Alex likes this movie, it seems necessarily to imply that Betty won't like this movie!  This directly contradicts the conclusion of the argument.   Negating (C) absolutely torpedoes this argument, so (C) must be an assumption.

BTW, notice that choice (A) constitutes anecdotal data: this can never be an assumption. Notice that the original argument has the strange logical jump from what Alex likes to what Betty likes, so using the Bridge Approach, one might have zeroed in on the correct assumption even before applying the Negation Test.  Nevertheless, the Negation Test always works, so it's always a good test of an assumption.

 

Summary

If any ideas in this post were new to you, you may want to give that argument at the top a second look before reading the solution below.  Here's another GMAT CR Assumption practice question:

- See more at: http://magoosh.com/gmat/2013/assumptions-and-the-negation-test-on-the-gmat/#sthash.ff3euCml.dpuf