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
In pre-school children, the percentage of thumb sucking is: 
Select and explain. 
A. <10%, B. 10-20%, C. <50%, D. >50%
D: >50%
Answer: D: >50% 
Explanation: 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.