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': "Horrock's appar is used for- \nSelect and explain. \nA. Wind velocity, B. Chlorine demand, C. Water contamination, D. Coliform count", 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Chlorine demand InstrumentUtilityHorrocks ApparatusChlorinator/ ChloronomeChloroscopeChlorine demand estimationMixing or regulating dose of chlorineMeasuring residual level of chlorine", 'role': 'assistant'}]