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.  Therefore, 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?

1. No part of a person’s brain that is damaged by a stroke ever recovers.
2. Impairment of linguistic capabilities does not occur in people who have not suffered any damage to any language center of the brain.
3. Strokes tend to impair linguistic capabilities more severely than does any other cause of damage to language centers in the brain.
4. 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.
5. 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.

   Answer: D

为何选D不选A

 recover from stroke是damage以后的事，如果a language center of the brain is damaged by a stroke, impairment of linguistic capabilities will definitely show in some way

文中所得到的结论必须有个前提就是stroke damages at least one of the language centers, 如果该前提不存在，也就是stroke可能对language centers 毫无影响，那么impairment of linguistic capabilities 就不会发生，这样也就不能断定language centers 是在左脑还是右脑