Which of the following statements about primary and secondary:

1-air pollutants are correct. Note that more than one statement can be correct and that you get minus points for incorrect answers.

2-Which of the following statements about primary and secondary air pollutants are correct. Note that more than one statement can be correct and that you get minus points for incorrect answers.

3-Primary air pollutants are not dangerous to human health.

4-Nitrogen dioxide (NO2) is only a primary air pollutant.

5-Nitrogen dioxide (NO2) is a primary and secondary air pollutant.

6-Secondary air pollutants are formed by the reaction of primary air pollutants with other substances in the air.