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
Most atypical (unique) tooth 
Select and explain. 
A. Primary lower 1st molar, B. Primary lower 2nd molar, C. Primary upper 1st molar, D. Primary upper 2nd molar
A: Primary lower 1st molar
Answer: A: Primary lower 1st molar 
Explanation: None