The secondary precursors of photochemical smog are

Fill in the blanks:
$(1)$ $A$ special cloud is formed over the Antarctica region in winter,which is called ..............
$(2)$ $A$ known source of pollution is called ..............
$(3)$ An unknown source of pollution is called ..............
$(4)$ The concentration of dissolved oxygen in water is ......... and the concentration of dissolved oxygen in air is ............

What do you mean by Biochemical Oxygen Demand $(BOD)$?

In municipal sewage,$BOD$ values (in $ppm$) are

Indiscriminate use of $DDT$ is undesirable because

Green chemistry means such reactions which