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: Physiology
[{'content': 'Citrate is a useful anticoagulant because of its ability to \nSelect and explain. \nA. Buffer basic groups of coagulation factors, B. Bind factor XII, C. Bind vitamin K, D. Chelate calcium', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]