This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': 'Which of the following is purification of water at large scale - a) Disinfection of well b) Rapid sand filterc)  Slow sand filterd)  Household purification \nSelect and explain. \nA. bc, B. c, C. ac, D. ad', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Purification of water \n\nPurification of water is of great importance in community.\nThe method of treatment to be employed depends upon the nature of raw water, and the desired standards of water quality.\nFor example ground water (deep well, tube well and springs) may need no treatment. On the other hand surface water (river) requires extensive treatment.\nPurification of water may be : -\n\nPurification on large scale\n\nStorage\nFiltration\n\n\nSlow sand (Biological filter)\nRapid sand (mechanical filter)\n\n\xa0 \xa0 \xa0 3.\xa0Disinfection\n\nChlorination\nOzonation\nUV irradiation\n\nPurification on small scale\n\nHousehold purification\n\n\nBoiling\nChemical disinfection\n\n\xa0 \xa0 \xa0 \xa02. Disinfection of wells', 'role': 'assistant'}]