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': 'Horrock’s apparatus estimates \nSelect and explain. \nA. Free chlorine, B. Chlorine demand, C. Both (a) + (b), D. Combined chlorine', 'role': 'user'}, {'content': "Answer: B \nExplanation: Used To find out the dose of bleaching powder required for disinfection of water, i.e. ‘Chlorine demand estimation of water.'", 'role': 'assistant'}]