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
What should be the diameter of post: 
Select and explain. 
A. 1/2 of root diameter., B. Same as root diameter., C. 1/3rd of root diameter., D. 1/4th of root diameter.
C: 1/3rd of root diameter.
Answer: C: 1/3rd of root diameter. 
Explanation: At least 1mm dentin should be present circumferentially around post.