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': 'Greening occurring when porcelains are fired on silver free alloys may be attributed to \nSelect and explain. \nA. Vaporization of silver from the walls of contaminated furnaces, B. Surface diffusion of silver from the marginal metal, C. More rapid silver diffusion in sodium containing glass, D. Silver ionization by porcelains with high oxygen potential', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]