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
Greening occurring when porcelains are fired on silver free alloys may be attributed to 
Select and explain. 
A. 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
A: Vaporization of silver from the walls of contaminated furnaces
Answer: A: Vaporization of silver from the walls of contaminated furnaces 
Explanation: None