The statement "Ozone in the atmosphere is decreased by chlorofluorocarbons $(CF_2Cl_2)$" is:

Surface water contains

Biochemical oxygen demand $(BOD)$ is a measure of organic materials present in water. $A$ $BOD$ value less than $5 \ ppm$ indicates a water sample to be:

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 .....

The pollutants which come directly into the air from sources are called primary pollutants. Primary pollutants are sometimes converted into secondary pollutants. Which of the following belongs to secondary air pollutants?

Which gas is detected in the smoke produced due to the incomplete combustion of petrol or diesel?