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': 'Recommended fluoride levels in the water should be \nSelect and explain. \nA. <2 ppm, B. <1 ppm, C. <1 g/litre, D. <2 g/litre', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]