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: Medicine
[{'content': 'The blood in the vessels normally does not clot because - \nSelect and explain. \nA. Vitamin K antagonists are present in plasma, B. Throbin has a positive feedback on plasminogel, C. Sodium citrate in plasma chelates calcium ions, D. Vascular endothelium is smooth and coated with glycocalyx', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]