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 of the following is the main constituent of porcelain: 
Select and explain. 
A. Feldspar, B. Kaolin, C. Quartz, D. Clay
A: Feldspar
Answer: A: Feldspar 
Explanation: None