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
Polishing agent for amalgam 
Select and explain. 
A. Tin oxide, B. Iron oxide, C. Alumina, D. Silica
A: Tin oxide
Answer: A: Tin oxide 
Explanation: TIN OXIDE:
This extremely fine abrasive material is used extensively as a polishing agent for polishing teeth and metallic restorations such as dental amalgams in the mouth. It is mixed with water, alcohol or glycerin to form a mildly abrasive paste.
Phillips Science of Dental Material 12th Edition Page. 245-246.