carbon monoxide (CO) nitrogen oxide (NO) methane (CH_{4}) ozone (O_{3})
I gave my students this multiple choice question: Secondary pollutants are produced in the environment by chemical reactions involving primary or emitted chemicals. Which of the following is a secondary pollutant?

Only one answer is correct among these 4 choices:
- carbon monoxide (CO)
- nitrogen oxide (NO)
- methane (CH_{4})
- ozone (O_{3})

Could you tell me which one is correct?

ozone (O_{3})