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
The most recent direct filling gold which produces the hardest surface on condensation is 
Select and explain. 
A. Mat gold, B. Encapsulated powered gold, C. Mat gold alloyed with calcium and wrapped in gold foil, D. Platinized gold
C: Mat gold alloyed with calcium and wrapped in gold foil
Answer: C: Mat gold alloyed with calcium and wrapped in gold foil 
Explanation: Mat gold made of electrolytic precipitate from gold, which is alloyed with a trace amount of calcium, can yield a further increase in hardness by dispersion strengthening without affecting the handling properties.