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': "Chlorine demand of water determined by – \nSelect and explain. \nA. Horrock's apparatus, B. Chloronometre, C. Berkefield filter, D. Doble Pot", 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]