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
[{'content': 'The biological oxygen demand indicates- \nSelect and explain. \nA. Organic matter, B. Bacterial content, C. Anaerobic bacteria, D. Chemicals', 'role': 'user'}, {'content': "Answer: A \nExplanation: 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.", 'role': 'assistant'}]