The BOD value of two samples of water are 4ppm and 18ppm, respectively and this indicates:

So₂ is the component of classical smog.

Complete reaction of acid rain is

$2S{O}_2+O_2+2H_{2}O\stackrel{}{\to }2H_{2}S{O}_4\left(acidrain\right)$  

BOD is the amount of oxygen required by bacteria to break down the organic matter present in a certain volume of sample of water.

Veronal is a barbiturate drug and it constitutes an important class of tranquilizers.

It is process in which nutrient enriched water bodies support a dense plant population which kills animal life by depriving it of oxygen and result in subsequent loss of biodiversity is known as Eutrophication.