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 retentive post among the following: 
Select and explain. 
A. Smooth tapered, B. Serrated tapered, C. Smooth parallel, D. Serrated parallel
D: Serrated parallel
Answer: D: Serrated parallel 
Explanation: Parallel-sided posts may distribute stress more evenly than tapered posts, which may have a wedging effect.
Serrated are more retentive than smoother surface.