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
[{'content': 'A radiopaque area is observed under a 3-month old silver amalgam filling. The base used most probably was: \nSelect and explain. \nA. Calcium hydroxide, B. ZOE, C. Formocresol, D. Zinc oxyphosphate', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]