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': 'Contact period of chlorination ? \nSelect and explain. \nA. 1 hour, B. 30 minutes, C. 1 1\\/2 hour, D. 2 hours', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Contact period for chlorination is one hour. The presence of free residual chlorine for a contact period of one hour is necesary to kill pathogens. The residual chlorine is 0.5 mg/L.Parks textbook of preventive and social medicine.K Park. Edition 23.Pg no: 715', 'role': 'assistant'}]