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
The biological oxygen demand indicates- 
Select and explain. 
A. Organic matter, B. Bacterial content, C. Anaerobic bacteria, D. Chemicals
A: Organic matter
Answer: A: Organic matter 
Explanation: Park's textbook of preventive and social medicine 23rd edition. *Aim of sewage treatment is to stabilize the organic matter,the strength of sewage is expressed in terms of biological oxygen demand.