What is characteristic of a thermal inversion? 

A)  Thermal inversions last only a few minutes to a few hours. 
B)  Thermal inversions exacerbate pollution problems. 
C)  Thermal inversion occurs when a layer of cold air prevents warm air from rising. 
D)  Normally, cool air near the Earth's surface expands and rises, carrying pollutants higher into the troposphere.

Thermal inversions exacerbate pollution problems by trapping pollutants near the Earth's surface, where they can accumulate and cause health problems.

Answer: C
Thermal inversion occurs when a layer of cold air is trapped near the Earth's surface, preventing warm air and pollutants from rising and dissipating.

Option A is incorrect because thermal inversions can last for days to weeks, not just a few minutes to a few hours. Option D is incorrect because it describes the opposite of a thermal inversion - normally, warm air rises and carries pollutants higher into the troposphere, but in a thermal inversion, cold air prevents this from happening.