What is Biochemical Oxygen Demand $(BOD)$?

An excess of ......... $ppm$ of $F^-$ ions in drinking water is responsible for a disease known as fluorosis.

Some statements are given:
$(S_1) \ CO_2$ is a greenhouse gas.
$(S_2)$ Classical smog contains smoke,fog,and a high concentration of $SO_2$.
$(S_3) \ DDT$ is biodegradable.
$(S_4)$ Drinking water should have a high $B.O.D.$
Which is the correct sequence of true $(T)$ or false $(F)$ regarding the above statements?

The delicate balance of $CO_2$ and $O_2$ is $NOT$ disturbed by:

The statement that is not true about ozone is:

During a school field trip,a botany student visited a beautiful pond in a village and collected some plants. He observed that villagers were washing clothes there,and household waste was spoiling its beauty in some areas. After a few years,he revisited the pond and found it completely covered with algae,emitting a foul smell,and the water had become unusable. Can you describe this condition of the pond?