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
Which is not present in porcelain: 
Select and explain. 
A. Silica, B. Feldspar, C. Calcium carbonate, D. Amorphous material
D: Amorphous material
Answer: D: Amorphous material 
Explanation: None