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': 'Preferential contamination of direct filling gold is done by: \nSelect and explain. \nA. Pure ethanol, B. 2% glutaraldehyde, C. 18% ammonia, D. 5% sodium hypochlorite', 'role': 'user'}, {'content': "Answer: C \nExplanation: After pellets of gold are rolled, they may be conveniently stored in a gold foil box, which is divided into\xa0labeled sections for various sizes of pellets.\xa0\nCylinders of foil and selected sizes of other types of gold\xa0also may be stored in the box. Preferential contamination is suggested by placing a damp cotton pellet\xa0dipped into 18% ammonia into each section of the box\nThis serves to prevent deleterious oxides from\xa0forming on the gold until it is used.\n\nKey Concept\nContamination of direct ﬁlling gold is done by 18% ammonia.\n\xa0\n\xa0\nSturdevant's art and science of operative dentistry. 7th edition page e69", 'role': 'assistant'}]