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A lake near a village suffered heavy mortality of fishes within a few days. Consider the following reasons for this

   I. Lots of urea and phosphate fertiliser were used in the crops in the vicinity. 

  II. The area was sprayed with DDT by an aircraft.

  III. The lake water turned green and stinky. 

 IV. Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis.

 Which two of the above were the main causes of fish mortality in the lake?

 (a) ll and lll

 (b) lll and lV

 (c) I and lll

 (d) I and ll

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Correct option (d) I and ll

Explanation:

 A lake near a village suffered heavy mortality of fishes within a few days, because lots of urea and phosphate fertiliser were used in the crops in the vicinity and the area was sprayed with DDT by an aircraft.

Inorganic phosphorus and nitrogen are responsible for the growth of algae. In polluted water these increase due to which algae increase greatly at the surface of water forming water bloom. Due to death of these algae their organic matter gets decomposed due to which oxygen gets depleted and aquatic animal dies.

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