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: Surgery
The main source for pulmonary embolism: 
Select and explain. 
A. Haemorrhoids, B. Deep Leg vein, C. Portal vein, D. None of the above
B: Deep Leg vein
Answer: B: Deep Leg vein 
Explanation: None