If the depletion of oxygen is found to be 2.5 mg/litre after incubating 2.5 ml of sewage diluted to 250 ml for 5 days at 20°C, B.O.D. of the sewage is

• 50 mg/l
• 100 mg/l
• 150 mg/l
• 200 mg/l
• 250 mg/l.