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': 'Radio-opacity in composite is due to presence of \nSelect and explain. \nA. Strontium, B. Fluorides, C. Silica, D. BisGMA', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]