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 is generally accepted as the first to erupt, and at about what mean age? 
Select and explain. 
A. Maxillary central incisor, 8 to 12 months., B. Maxillary central incisor, 7 to 9 months., C. Mandibular central incisor, 7 to 9 months., D. Mandibular central incisor, 8 to 10 months.
C: Mandibular central incisor, 7 to 9 months.
Answer: C: Mandibular central incisor, 7 to 9 months. 
Explanation: Eruption for the primary mandibular central incisor may be given as 6 months (Woelfel and Scheid, 2002); 6.5 months (Kraus et al., 1969); 7.5 months(Charlick et al.); or 8 (6 to 10) months (Ash and
Nelson, 2003).