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: Pharmacology
[{'content': 'Heparin is the most widely used antithrombotic agent. It acts by activating - \nSelect and explain. \nA. Plasmin, B. Antithrombin III, C. Fibrinolysin, D. Factor X', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Antithrombin III", 'role': 'assistant'}]