QUESTION:
Which line shows how the oxygen concentration of the water changes after excess fertiliser has entered a stream?

OPTIONS:
A: more oxygen downstream
B: more oxygen near fertiliser run-off and downstream but less oxygen in the middle
C: same oxygen everywhere
D: less oxygen near fertiliser run-off and downstream but more oxygen in the middle

NOTE:
The correct answer is B but I need an explanation as to why. I thought the answer was A and got it wrong.