question_answer 1)

  Sewage containing organic waste should not be disposed in water bodies because it causes major water pollution. Fishes in such polluted water die because of: (a) large number of mosquitoes                                                (b) increase in the amount of dissolved oxygen (c) decrease in the amount of dissolved oxygen in water (d) clogging of gills by mud

Answer:

  (c) Dissolved oxygen is essential for sustaining life in any aquatic system. Organic waste is oxygen consuming waste. It is decomposed by bacteria which in turn decreases oxygen from water. Fishes in such a polluted water do not survive.