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': 'Main disadvantage of PFM restoration is: \nSelect and explain. \nA. Excessive tooth reduction, B. Porcelain is very brittle, C. PFM restoration is unaesthetic, D. All of the above', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]