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. Horrocks apparatus, B. Chloronometre, C. Berkefield filter, D. Double Pot', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Horrocks apparatus', 'role': 'assistant'}]