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. Ttherefore, 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.", 'The chemicals are not commonly used as cleaners or pesticides in houses and apartment buildings in Renston.', '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.']