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: Pathology
[{'content': 'The most common site of origin for venous thrombi leading to pulmonary embolism is \nSelect and explain. \nA. Ascending aorta, B. Portal vein, C. Deep leg veins, D. Right atrium', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]