A Cave Study, a Confound, and the Best-Supported Claim
A passage reads: 'Researchers placed identical ceramic tiles in four caves. In the dampest cave, tiles developed a mineral crust within eight months; tiles in three drier caves showed no crust after two years. The authors conclude that humidity, not temperature, controls crust formation. Their figure shows that the damp cave was also the only one with limestone walls, which can release minerals into the air.' A question asks: 'Which statement is best supported by the passage?' A student picks 'Temperature plays no role in crust formation.' Another picks 'The evidence is consistent with humidity causing crusts but does not rule out limestone.'
1. Evaluate the two answer choices. Which is better supported, and why?
Read the worked answer
The second choice is better. The study compared caves, not controlled humidity levels, and the damp cave differed in a second way: limestone walls. That is a confound, since limestone can release crust-forming minerals. 'Temperature plays no role' is an overreach because the passage reports no temperature manipulation or measurements; it only reports the authors' conclusion. The safest supported statement is the cautious one.
2. What is the function of the sentence about limestone walls in the passage?
Read the worked answer
It functions as a limitation on the causal claim. By noting that the damp cave was also the only limestone cave, the passage supplies an alternative source of crust-forming minerals. It does not refute the humidity hypothesis, but it weakens the inference that humidity alone caused the result.
3. What additional experiment would most strengthen the humidity conclusion?
Read the worked answer
Repeat the experiment in a damp cave without limestone walls, or control humidity in a lab while holding temperature and wall material constant. If crusts still form under high humidity alone, the limestone alternative is ruled out and the humidity explanation is strengthened. Adding more dry caves would not address the confound.