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
Main mechanism of action of heparin Is to prevent: 
Select and explain. 
A. Conversion of fibrinogen to fibrin, B. Conversion of prothrombin to thrombin, C. PTC to PTA, D. Factor VIIa inhibition
B: Conversion of prothrombin to thrombin
Answer: B: Conversion of prothrombin to thrombin 
Explanation: None