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: Surgery
Most difficult maxillary tooth to anaesthetize by infiltration is : 
Select and explain. 
A. 1st molar, B. 1st premolar, C. Canine, D. 3rd molar
A: 1st molar
Answer: A: 1st molar 
Explanation: None