請教GWD7-Q5,謝謝

GWD7-Q5:

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?

1. The number of
        school nurses employed by Renston’s elementary schools has not decreased
        over the past ten years.
2. Children who are
        allergic to the chemicals are no more likely than other children to have
        allergies to other substances.
3. 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.
4. The chemicals
        are not commonly used as cleaners or pesticides in houses and apartment
        buildings in Renston.
5. Children
        attending elementary school do not make up a larger proportion of
        Renston’s population now than they did ten years ago.    這題答案是C,請問B錯在哪?C又對在哪?