The conclusion of the argument is "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". So we have to assume that there can't be any other reason for the increase in the proportion of children sent for treatment other than these reasons.
Hence C is the right answer as if "students are more likely to be sent to a school nurse now than they were ten years ago", it
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Hence C is the right answer as if "students are more likely to be sent to a school nurse now than they were ten years ago", it
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