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
Amalgam contributes to etiology of chronic periodontal disease overlong because it acts as: 
Select and explain. 
A. A mechanical irritant, B. Plaque retaining area, C. Nondeficiency tooth surface, D. overcontoured tooth surface
B: Plaque retaining area
Answer: B: Plaque retaining area 
Explanation: None