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The given graph shows changes in concentration of two substances X and Y in a river polluted by sewage. Select the correct option regarding X and Y.

A)  Eutrophication occurs in river due to the enrichment of substance Y.

B)  BOD of river is comparatively lower when concentration of substance X is reduced.

C)  Substance Y helps in the recovery of river from sewage pollution.

D)  None of these.                                                             '