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 commonly retained primary tooth is: 
Select and explain. 
A. Maxillary first molar., B. Mandibular first molar., C. Maxillary lateral incisor., D. Mandibular 2nd molar.
D: Mandibular 2nd molar.
Answer: D: Mandibular 2nd molar. 
Explanation: None