(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 ?
$\begin {array} {1 1} (1)\;a,b & \quad (2)\;b,c \\ (3)\;c,d & \quad (4)\;a,c \end {array}$
