$A$ 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?

• A
  $2$ and $3$
• B
  $3$ and $4$
• C
  $1$ and $3$
• D
  $1$ and $2$