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': 'In a metal ceramic crown for a FPD the function of tin, indium which are readily oxidized minor constituents is to: \nSelect and explain. \nA. Improve bonding, B. Decrease porosity, C. They react with the porcelain opaque layer, which masks the metal, D. Improves the thermal expansion between the metal and porcelain', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]