Passage Summary: Charles thinks recessions lead to cleaner air because fewer people drive to work; Darla counters by noting that recessions make people drive older, dirtier cars.

Reasoning: Darla points out that during recessions, people keep older cars longer, and older cars emit more pollutants than newer ones.

Analysis: Darla’s role is to provide a counter-consideration that undermines Charles’s causal claim. She doesn't necessarily deny that commuting decreases, but she introduces an alternative variable—vehicle age and emission efficiency—that could negate Charles's predicted outcome. To identify this method, look for a description of a speaker introducing a factor that offsets or outweighs the evidence provided by the first speaker.