Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
Use the PPT to answer.
What are primary pollutants?
What are secondary pollutants?