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.


Answer: (iii)
Solution:
 (i) Large number of mosquitoes. – This option is incorrect. Mosquitoes are not directly responsible for the death of fishes in polluted water.
(ii) Increase in the amount of dissolved oxygen. – This option is also incorrect. While an increase in dissolved oxygen can be beneficial for aquatic life, it is not the reason why fishes in polluted water die.
(iii) Decrease in the amount of dissolved oxygen in water. – This option is correct. When sewage containing organic waste is disposed in water bodies, the organic matter is decomposed by bacteria, which consume oxygen during the process. This can lead to a decrease in the amount of dissolved oxygen in the water, which can be harmful for aquatic life, including fishes. If the oxygen level drops too low, fishes may suffocate and die.
(iv) Clogging of gills by mud. – This option is also incorrect. While clogging of gills by mud or other particles can be harmful for fishes, it is not the primary reason why fishes in polluted water die.

Therefore, the correct answer is option (iii), which states that fishes in polluted water die because of a decrease in the amount of dissolved oxygen in the water.

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