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': 'All are features of septic tank except: \nSelect and explain. \nA. Ideal retention period-48 hours, B. Minimum capacity-500 gallons, C. Aerobic oxidation takes place outside, D. Sludge is solids setting down', 'role': 'user'}, {'content': 'Answer: A \nExplanation: SEPTIC TANK: Is a water-tight masonary tank into which household sewage is admitted for treatment Is a satisfactory method of disposing liquid and excreta wastes from individuals dwellings, small groups of houses or institutions which have &;adequate water supply but donot have access to a public sewerage system&; Design features of a septic tank: Ideal retention period: 24 hours Steps of purification in a septic tank: Anaerobic digestion: Takes place in septic tank proper Aerobic oxidation: Takes place in sub-soil (outside septic tank) Ref: Park 25th edition &; Pgno: 797', 'role': 'assistant'}]