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