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 heights of contour of the distal surfaces of permanent mandibular central incisors are located in which coronal third? 
Select and explain. 
A. Middle., B. Cervical., C. Occlusal., D. Incisal.
D: Incisal.
Answer: D: Incisal. 
Explanation: Height of contour on distal surface of permanent mandibular central incisors is at the incisal third).