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 fourth root canal if present in a maxillary 1st molar
is usually present in: 
Select and explain. 
A. Mesiolingual root, B. Mesiobuccal, C. Palatal root, D. Distal root
B: Mesiobuccal
Answer: B: Mesiobuccal 
Explanation: None