Question

tall smokestacks were built due to the theory \dilution is the solution to pollution.\ it caused local air quality to improve, but what problems did it cause? decreased air quality downwind improved global air quality

Explanation:

Tall smokestacks disperse pollutants higher, so local air improves. But pollutants travel downwind, reducing air quality there. "Improved global air quality" is wrong as pollution is just relocated, not eliminated.

Answer:

Decreased air quality downwind