Which of the following is called a secondary air pollutant?

Air pollutants are classified into two categories:
* Primary Pollutants: Emitted directly into the atmosphere from sources (e.g., Carbon monoxide ($\text{CO}$), Ammonia (${\text{NH}}_3$), and Nitrogen dioxide (${\text{NO}}_2$)).
Secondary Pollutants: Formed in the atmosphere through chemical reactions between primary pollutants and environmental factors. *Ozone (${\text{O}}_3$)** is a classic secondary pollutant created when nitrogen oxides (${\text{NO}}_x$) and volatile organic compounds ($\text{VOCs}$) react in the presence of sunlight.