Passage Summary: A school changed its AC to save money, which made the air drier; shortly after, more kids went to the nurse, so the school blames the dry air for making them sick.

Conclusion: A drop in air humidity can cause people to become sick.

Reasoning: After the school's humidity decreased due to a new AC system, there was a significant spike in students visiting the nurse.

Analysis: This is a classic causal argument where the author assumes 'after this, therefore because of this.' For the conclusion to hold, we must assume no other factor, like a flu outbreak or a change in school policy, caused the spike in nurse visits at that exact time. We also need to assume that the people visiting the nurse were actually ill, rather than visiting for unrelated reasons. Look for an answer that eliminates an alternative cause for the sudden increase in nurse visits.