Which of the following is an air pollutant?

What is the volume percentage of nitrogen gas in the atmosphere (in $\%$)?

Which of the following statements is correct?

Given below are two statements :
Statement $I$ : If $BOD$ is $4\,ppm$ and dissolved oxygen is $8\,ppm$,then it is a good quality water.
Statement $II$ : If the concentration of zinc and nitrate salts are $5\,ppm$ each,then it can be a good quality water.
In the light of the above statements,choose the most appropriate answer from the options given below :

What is the percentage contribution of $CO$ to the total atmospheric concentration due to forest fires?

Which of the following statements about photochemical smog is wrong?