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
Most constant and valuable trait to differentiate among maxillary 1st , 2nd and 3rd molars is 
Select and explain. 
A. Relative position of DL groove, B. Relative position of ML groove, C. ML cusp size, D. Oblique ridge size
A: Relative position of DL groove
Answer: A: Relative position of DL groove 
Explanation: Relative position of DL groove is most constant and valuable trait. As DL cusp becomes smaller as we move distally from 1st molar to 3rd molar.