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
How many osseous walls are present in one walled  vertical defects? 
Select and explain. 
A. One wall present, B. Two walls present, C. There walls present, D. Four walls present
A: One wall present
Answer: A: One wall present 
Explanation: None