This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Biochemical oxygen demand is calculated to know ----- wastes - 
Select and explain. 
A. Organic, B. Inorganic, C. Total solid, D. None
A: Organic
Answer: A: Organic 
Explanation: Biochemical oxygen demand is an impoant test done on sewage. It is defined as the amount of oxygen adsorbed by a sample of sewage during a specified period, generally 5 days, at a specified temperature for the aerobic destruction or use of organic matter by living organisms. If BOD more than 300mg/L - strong sewage Less than 100mg/L - weak sewage Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 823