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
Landmark for pup chamber opening is : 
Select and explain. 
A. CEJ, B. Cervical third of crown, C. Level of alveolar bone, D. Enamel
A: CEJ
Answer: A: CEJ 
Explanation: Krasner and Rankow determined that the CEJ is the most important anatomic landmark for determining the location of pulp chambers and root canal orifices. They demonstrated  that  specific  and  consistent  pulp  chamber floor and wall anatomy exist and proposed laws for assisting  clinicians  to  identify  canal  morphology.
Ref: Grossman endodotic practice 13th ed page no 248