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 most common source of pulmonary embolism \nSelect and explain. \nA. Amniotic fluid embolism, B. Renal artery embolism, C. Large veins of leg, D. Cardio thoracic surgery', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]