Which of the following is responsible for the depletion of ozone in the stratosphere?

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 :

Green plants use carbon dioxide during photosynthesis and release oxygen,yet carbon dioxide is considered responsible for the greenhouse effect. Why?

$A$ large number of fish are suddenly found floating dead on a lake. There is no evidence of toxic dumping,but you find an abundance of phytoplankton. Suggest a reason for the fish kill.

The conversion of sulfur dioxide to sulfur trioxide in the absence of a catalyst is a slow process. However,this process occurs easily in the atmosphere. Describe how this happens and write the chemical equations for the formation of $SO_3$ from $SO_2$.

How is biogas produced?