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
Which of the following post is most difficult to bond in the root canal: 
Select and explain. 
A. Zirconia ceramic post., B. Carbon fiber reinforced post., C. Polyethylene fiber reinforced post., D. Glass fiber reinforced post.
A: Zirconia ceramic post.
Answer: A: Zirconia ceramic post. 
Explanation: It is because zirconia cannot be etched.