For most people, the left half of the brain controls linguistic capabilities, but some people have their language centers in the right half. When a language center of the brain is damaged, for example by a stroke, linguistic capabilities are impaired in some way. Ttherefore, people who have suffered a serious stroke on the left side of the brain without suffering any such impairment must have their language centers in the right half. Which of the following is an assumption on which the reasoning in the argument above depends? ["It is impossible to determine which side of the brain contains a person's language centers if the person has not suffered damage to either side of the brain.", "No part of a person's brain that is damaged by a stroke ever recovers.", 'Strokes tend to impair linguistic capabilities more severely than does any other cause of damage to language centers in the brain.', 'If there are language centers on the left side of the brain, any serious stroke affecting that side of the brain damages at least one of them.']