The ozone layer forms naturally by

The $COD$ (Chemical Oxygen Demand) of a water sample is determined using:

Write down the reactions involved during the formation of photochemical smog.

Match the activities given in List-$I$ with the pollution produced in List-$II$.

List-$I$	List-$II$
$(A)$ Burning sulfur-containing waste releases gases into the atmosphere.	$(1)$ Water pollution
$(B)$ Use of carbonates as pesticides	$(2)$ Photochemical smog,damage to plant life,corrosion of buildings,respiratory problems,water pollution.
$(C)$ Using synthetic detergents for washing clothes	$(3)$ Damage to the ozone layer
$(D)$ Releasing smoke from vehicles and factories into the atmosphere.	$(4)$ Neurological disorders in humans.
$(E)$ Using chlorofluorocarbon compounds to clean computer parts.	$(5)$ Classical smog,acid rain,water pollution,respiratory problems,damage to buildings,metal corrosion.

Given below are two statements:
Statement $I:$ Classical smog occurs in cool humid climate. It is a reducing mixture of smoke,fog and sulphur dioxide.
Statement $II:$ Photochemical smog has components,ozone,nitric oxide,acrolein,formaldehyde,$PAN$ etc.
In the light of above statements,choose the most appropriate answer from the options given below:

The component that is present in both the troposphere and the stratosphere is $......$.