How is ground-level ozone formed and why is it considered a pollutant?

• A. O3 forms from sunlight-driven reactions between NOx and VOCs in the lower atmosphere; it irritates lungs, reduces lung function, and damages vegetation.
• B. O3 forms in the stratosphere and has no health effects.
• C. O3 is directly emitted by vehicles and factories but does not form in the atmosphere.
• D. O3 is formed by combustion of methane and is beneficial to vegetation.

Answer: (C)