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
The opacity in ceramics is achieved by adding: 
Select and explain. 
A. Boric oxide, B. Copper oxide, C. Silica, D. Titanium oxide
D: Titanium oxide
Answer: D: Titanium oxide 
Explanation: None