Correct Answer - A
Effluent discharge mixing into a river will result in rise of BOD because decomposer organisms consume a lot of oxygen. If sewage quantity is large, the whole of dissolved oxygen may be consumed leaving nothe for respiration of fish and other clean water organisms. They, therfore, get killed. However, as sewage is decomposed, there is a gradual rise in dissolved oxygen downstream. Fish and other clean water organisms reappear indicating the recovery of river from sewage pollution. Meanwhile, number of pollution resistant species increases in the stream.
