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
Shape of the central groove in Mandibular 2nd premolar is 
Select and explain. 
A. Y shaped, B. H shaped, C. U shaped, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None