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': 'The opacity in ceramics is achieved by adding: \nSelect and explain. \nA. Boric oxide, B. Copper oxide, C. Silica, D. Titanium oxide', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]