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': 'In pre-school children, the percentage of thumb sucking is: \nSelect and explain. \nA. <10%, B. 10-20%, C. <50%, D. >50%', 'role': 'user'}, {'content': 'Answer: D \nExplanation: According to a longitudinal study, which took data from a good number of studies, the average rate of non-nutritive sucking is about 70-80% in the preschool age.', 'role': 'assistant'}]