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
The key to occlusion is: 
Select and explain. 
A. Maxillary permanent first molar, B. Maxillary permanent second molar, C. Maxillary primary first molar, D. Maxillary primary second molar
A: Maxillary permanent first molar
Answer: A: Maxillary permanent first molar 
Explanation: None