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 20 mg/L and 400 mg/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 ?