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': 'Past contamination of water is indicated by – \nSelect and explain. \nA. Free and saline ammonia, B. Dissolved H2S, C. Nitrites, D. Nitrates', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]