$A$ man uses municipal water. Due to water scarcity,he starts using groundwater. He experiences a laxative effect. What could be the reason for this?

Methyl mercaptans are generated as waste from which industry?

The $BOD$ values for pure water and highly polluted water are respectively

Depletion of ozone layer is caused by

The possibility of photochemical smog formation will be minimum at

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 ............