3.0 ml of raw sewage is diluted to 300 ml. The D.O. concentration of the diluted sample at the beginning of the test was 8 mg/l. After 5 day-incubation at 20C, the D.O. concentration was 5 mg/l. The BOD of raw sewerage is______________________?
A. 300 mg/l

B. 400 mg/l

C. 200 mg/l

D. 100 mg/l

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