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': 'Most commonly used retarder in gypsum: \nSelect and explain. \nA. Na2 SO4, B. Nacl, C. Citrates, D. K2 SO4', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]