What kind of process is integrated organic farming?

$A$ lake near a village suffered heavy mortality of fishes within a few days. Consider the following reasons for this:
$1.$ Lots of urea and phosphate fertilizer were used in the crops in the vicinity.
$2.$ The area was sprayed with $DDT$ by an aircraft.
$3.$ The lake water turned green and stinky.
$4.$ 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?

Identify the correct statement regarding Eutrophication.

For which major rivers in India has the Ministry of Environment formulated an action plan?

All statements regarding $BOD$ (Biochemical Oxygen Demand) are correct,except:

The $CO_2$ content of the atmosphere has increased in the last $150$ years from: