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
Horrock's appar is used for- 
Select and explain. 
A. Wind velocity, B. Chlorine demand, C. Water contamination, D. Coliform count
B: Chlorine demand
Answer: B: Chlorine demand 
Explanation: Ans. is 'b' i.e., Chlorine demand InstrumentUtilityHorrocks ApparatusChlorinator/ ChloronomeChloroscopeChlorine demand estimationMixing or regulating dose of chlorineMeasuring residual level of chlorine