Which of the following is a secondary pollutant?

Primary pollutants are emitted directly from sources (e.g., ${\text{SO}}_2,\text{CO},{\text{CO}}_2$). Secondary pollutants are formed in the atmosphere by chemical reactions between primary pollutants and other atmospheric components. Ozone (${\text{O}}_3$) is a secondary pollutant formed by the reaction of ${\text{NO}}_x$ and volatile organic compounds in the presence of sunlight.