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
Ferrule can be defined as: 
Select and explain. 
A. Vertical band of metal that supports and strengthen tooth., B. Horizontal band of metal that supports and strengthen tooth., C. Thickness of remaining dentin around the post., D. Diameter of remaining dentin around the post.
A: Vertical band of metal that supports and strengthen tooth.
Answer: A: Vertical band of metal that supports and strengthen tooth. 
Explanation: None