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: Dental
[{'content': 'Thumb sucking is normal upto: \nSelect and explain. \nA. upto 1½ years, B. 3½ - 4 years, C. 6 - 8 years, D. 4 - 6 years', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]