Which statement best differentiates photochemical smog from gray smog?

• A. Gray smog forms from sunlight and NOx.
• B. Photochemical smog stems from coal burning.
• C. Photochemical smog forms from sunlight reacting with NOx and VOCs, while gray smog stems from coal burning and sulfur compounds.
• D. Both smogs involve only indoor pollutants.

Answer: (C)

The main distinction is how each type of smog forms: photochemical smog is driven by sunlight causing reactions between nitrogen oxides and volatile organic compounds, producing ozone and other oxidants in sunny, polluted urban air; gray smog (industrial or sulfurous smog) comes from burning coal and sulfur-containing fuels, releasing sulfur dioxide and particulates that create a gray haze, especially in cooler, humid conditions. The statement that photochemical smog forms from sunlight reacting with NOx and VOCs, while gray smog stems from coal burning and sulfur compounds, captures these differing sources and mechanisms. The other options mix up the sources or conditions (for example, attributing gray smog to sunlight and NOx, or photochemical smog to coal burning, or suggesting indoor origins), which don’t fit how these smogs actually form.