The author concludes that "people who have suffered a serious stroke on the left side of the brain without suffering any [linguistic] impairment must have their language centers in the right half."

He/she bases this conclusion on the following reasoning:

• People either have language centers in the right or left half of their brain
• When a linguistic center is damaged, linguistic capabilities are impaired.

We need to find an assumption on which the argument depends -- in other words, the correct answer choice MUST be true in order for the conclusion to follow from the evidence presented.

Take a look at (D):

The author's conclusion centers on people who have had a serious stroke on the left side of the brain (which does not sound fun). These people suffered this stroke, but their linguistic capabilities were not impaired -- yay! From this, the author concludes that these people's language centers must not have been in the left side of the brain at all. Instead, those centers must be in the right side of the brain.

But wait -- what does a "serious stroke" in half of the brain really mean? Would a stroke like that actually damage everything in that side of the brain? Or could some brain functions exist in that side of the brain, but survive the stroke without being impaired?

To conclude that the language centers MUST be in the right side of the brain, the author has to assume that a serious, left-side stroke would absolutely, without a doubt damage some language centers if they were indeed located in the left side of the brain. Otherwise, it is entirely possible for a person with left-side language centers to have a serious left-side stroke without any linguistic impairment -- maybe that stroke just happened to miss all of the language centers in the left side.

(D) MUST be true in order for the author's conclusion to hold, so (D) is an assumption on which the argument depends.


Hmm. (A) gives us information about recovering from a stroke. If you look back at the passage, the author doesn't specify whether the impairment caused by a stroke is permanent -- he/she just says that "when a language center of the brain is damaged, for example by a stroke, linguistic capabilities are impaired in some way." Whether that damage is temporary or permanent does not impact the author's argument at all.

Because the information in (A) doesn't HAVE to be true for the author's argument to hold, it is not an assumption on which the argument depends. Eliminate (A).