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': 'Purification of water at large scale is done by all except \nSelect and explain. \nA. Disinfection of well, B. Rapid sand filter, C. Slow sand filter, D. Storage', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Purification of water on a large scale Storage Filtration by slow sand or biological filters rapid sand or mechanical filters 3. Disinfection Purification of water on a small scale Household purification of water Disinfection of wells (refer pg:771 park 25th edition)', 'role': 'assistant'}]