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': 'Old contamination of water is indicated by – \nSelect and explain. \nA. Nitrites, B. Nitrates, C. Ammonia, D. Chlorides', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]