Final answer:
The subject of this question is Geography, and during a five-day period, a temperature inversion layer trapped a toxic brew of air pollutants over the town of Donora, Pennsylvania, causing health issues and fatalities.
Step-by-step explanation:
The subject of this question is Geography because it pertains to the location and environmental conditions of a specific town.
During a five-day period, a temperature inversion layer trapped a toxic brew of air pollutants over the town of Donora, Pennsylvania, killing some twenty people and sending half the town's population to the hospital.