Passage Summary: People complained about dirty air, which led to new laws, and those laws are the only reason why the air in once-polluted cities is cleaner today.

Reasoning: Public pressure led to stricter emission laws, and these laws were a necessary requirement for the air quality improvements seen in formerly polluted cities over the last 30 years.

Analysis: This 'Must be True' question hinges on a conditional relationship established in the final sentence. The phrase 'would not have happened without' indicates that the stricter regulations were a necessary condition for the improvement in air quality. Consequently, if a city experienced a significant improvement in air quality during this period, we can conclude that stricter regulations must have been in place. Look for an answer that treats the regulations as a mandatory requirement for the observed cleanup.