This is a causal arg. Hence the defender will avoid the alternate cause.

Since the number of childern being sent for treatment is on the rise the counterargument can be - more children are being sent for treatment to nurse hence more are being reported sick. The defender will avoid such a controversy and prevent the arg from getting weakened. Hence C

@ulm, please negate choice B and see the result.
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C

C

Well indeed it says that the number of children going to nurses now has increased. The next sentence says: Therefore, EITHER, its due to more exposure or the kids just became more sensitive THAN 10 years ago..

In that way C does fit best..

Question Stem
Which of the following is an assumption on which the argument depends?
Assumption Question- Ask the question - what is required so that the conclusion is true. Also use the negation test on last 2 option choices.

Argument
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 school children 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.

Pre-thinking
More children became aware of the symptoms and reported the same. So they were sent to the nurses.

Answer Choices

A. The number of school nurses employed by Renston’s elementary schools has not decreased over the past ten years.
INCORRECT ANSWER - This answer could have been correct if the argument did not mention "PROPORTION". Even if the nurses reduced, the proportion calculation would still remain the same -
\frac{No of students sent}{Total no of students}

B. Children who are allergic to the chemicals are no more likely than other children to have allergies to other substances.
INCORRECT ANSWER - "other substances" is outside the scope of the argument

C. 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.
CORRECT ANSWER - If the chance that children are sent to the nurse is same, then the conclusion is true.

Negation Test - Children who have allergic reactions to the chemicals are more likely to be sent to a school nurse now than they were ten years ago - So this statement rules out both the options in the conclusion. So this is the CORRECT ANSWER.

D. The chemicals are not commonly used as cleaners or pesticides in houses and apartment buildings in Renston.
INCORRECT ANSWER - If not used commonly then the chemical may not be a reason. It weakens the conclusion.

E. Children attending elementary school do not make up a larger proportion of Renston’s population now than they did ten years ago.
INCORRECT ANSWER - "larger proportion of Renston’s population" - this does not matter. The argument is not talking about this but the proportion of students sent to nurses.
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Regards,
gmatsuperstar

Paraphrasing the main point: According to nurses, something has changed in the past 10 years to make schoolchildren more sensitive to chemicals.
Paraphrasing the conclusion: 1) more chemicals, or 2) more sensitivity to the chemicals

A. I saw this as irrelevant. The number of nurses doesn't matter to establishing a proportion. The main point and conclusion can still be valid even with less nurses.
B. Comparing chemical allergies to other allergies is not within the stated argument. Also irrelevant.
C. This is correct. It offers an alternative explanation of why nurses would see an increase in the number of students sent to them with chemical allergies even if there weren't more chemicals or if the sensitivities had not changed.
D. The location of the exposure to chemicals is not relevant to the argument or conclusion. If D were true and chemicals were commonly used elsewhere, that would actually strengthen argument 1) as a reason for increased exposure.
E. Irrelevant. Schoolchildren to overall population proportion has nothing to do with the passage.