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
Which primary tooth generally erupts last? 
Select and explain. 
A. Maxillary second molar., B. Mandibular second molar., C. Maxillary canine., D. Mandibular canine.
A: Maxillary second molar.
Answer: A: Maxillary second molar. 
Explanation: Textbook chronologies indicate that the maxillary primary second molar is most commonly the last  to erupt:

24 months (Woelfel and Scheid, 2002)
20 to 30 months (Kraus et al., 1969)
25 to 33 months (Ash and Nelson, 2003)