the sewage one HAS TO be "number of bacteria increase" as the first stage, so i think it started with 4-x-x-x as the order of events...i4got if it was C or D but when sewage is let in2 a river first the no of bacteria increase thats obvious, THEN they start depleting oxygen levels, fish die, and as a result, more anaerobic organisms (hu can survive without O2) are remaining.