Passage Summary: Poison ivy and oak produce an oil that hurts humans but doesn't seem to bother other animals much; wood rats even build homes with it.

Conclusion: Urushiol likely did not develop as a defensive mechanism for plants.

Reasoning: Only humans have a painful reaction to it, while other animals have mild or no reactions, and some even use the plants for nesting.

Analysis: The author assumes that if a trait evolved for defense, it should be effective against more than just humans. To strengthen this, we need to show that the animals that interact with these plants in the wild are not deterred by the oil. Look for an answer that suggests the oil fails to protect the plant from its actual natural herbivores or that humans were not a relevant threat during the plant's evolution.