Microbes in Human Welfare

Three water samples namely river water, untreated sewage water and
secondary effluent discharged from a sewage treatment plant were
subjected to BOD test. The samples were labelled A, B and C; but the
laboratory attendant did not note which was which. The BOD values
of the three samples A, B and C were recorded as 20mg/L, 8mg/L and
400mg/L, respectively. Which sample of the water is most polluted?
Can you assign the correct label to each assuming the river water is
relatively clean?