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': 'All included in WHO criteria for water quality except \nSelect and explain. \nA. Hardness of water below 3000 mg/l, B. Lead below 0.01 mg/l, C. Nitrates <3 mg/l, D. Zero infections micro - organisms', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Nitrates < 45 mg/l\nNitrites <3 mg /l', 'role': 'assistant'}]