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Exposure to certain chemicals commonly used in elementary schools as cleaners or pesticides causes allergic reactions in some children. Elementary school nurses in Renston report that the proportion of schoolchildren sent to them for treatment of allergic reactions to those chemicals has increased significantly over the past ten years. Therefore, either Renston’s schoolchildren have been exposed to greater quantities of the chemicals, or they are more sensitive to them than schoolchildren were ten years ago.

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

The number of school nurses employed by Renston’s elementary schools has not decreased over the past ten years.
Children who are allergic to the chemicals are no more likely than other children to have allergies to other substances.
Children who have allergic reactions to the chemicals are not more likely to be sent to a school nurse now than they were ten years ago.
The chemicals are not commonly used as cleaners or pesticides in houses and apartment buildings in Renston.
Children attending elementary school do not make up a larger proportion of Renston’s population now than they did ten years ago.

请教各位NN，能否把此题的意思用中文解释一下，实在不懂为何选C.多谢！！！因本人下周就要考，所以很急。谢谢。

题目的前提是Elementary school nurses in Renston report that the proportion of schoolchildren sent to them for treatment of allergic reactions to those chemicals has increased significantly over the past ten years.即10年来送到护士那儿的过敏的小孩越来越多了。题目的结论是Therefore, either Renston’s schoolchildren have been exposed to greater quantities of the chemicals, or they are more sensitive to them than schoolchildren were ten years ago.即题目认为原因要么是小孩接触了更多的化学物质（过敏源），要么是因为小孩比十年前更容易过敏了。问题目的假设是什么。

GMAT假设的题目有两种类型：一种是填补前提和结论之间的概念上的漏洞，另一种是把可能削弱结论的其它因素排除。本题属于第二种。对于这种题目，把正确答案的选项取反将极大的削弱原题的结论。

因此，对于本题，只要在选项中找到一种被排除了的可能同样导致结论的其它原因就可以了。这就是C所说的，即不是因为现在过敏的小孩比十年前更有可能被送到护士那里。