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
Thumb sucking is normal upto: 
Select and explain. 
A. upto 1½ years, B. 3½ - 4 years, C. 6 - 8 years, D. 4 - 6 years
B: 3½ - 4 years
Answer: B: 3½ - 4 years 
Explanation: None