Sewage containing organic waste should not be disposed in water bodies because it causes major water pollution. Fishes in such a polluted water die because of
(i) Large number of mosquitoes.
(ii) Increase in the amount of dissolved oxygen.
(iii) Decrease in the amount of dissolved oxygen in water.
(iv) Clogging of gills by mud.
The answer is the option (iii) Decrease in the amount of dissolved oxygen in the water.
Explanation: Microorganisms oxidise organic waste in the presence of dissolved oxygen present in water; thus, decreasing the amount of dissolved oxygen present in water.