Charles: During recessions unemployment typically rises  Thus, during a recession air pollution due to automobile exhaust decreases, since fewer people commute in cars to jobs and so cars emitting pollutants into the air are used less  Darla: Why think that air pollution would decrease
During a recession fewer people can afford to buy new cars, and cars tend to emit more pollutants as they get older
It presents an argument showing that the premises in Charles' s argument support an absurd conclusion that Charles has overlooked.