Smog is primarily caused by the presence of which of the following?

Which of the following statements is correct?

Give the details of the different layers of the atmosphere and briefly describe the substances present in them.

Fill in the blanks:
$(1)$ The substance that spreads pollution is called .......
$(2)$ The full form of $DDT$ is .......
$(3)$ The area from $10 \ km$ to $50 \ km$ above sea level is called .........
$(4)$ Harmful ultraviolet radiation from the sun reaching the earth is blocked by .....

Which of the following is significant for water pollution?

Which air pollutant is responsible for the Bhopal gas tragedy?