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: Physiology
Which of the following anticoagulants prevent clotting of blood in the normal vascular system: 
Select and explain. 
A. Antithrombin III, B. Plasminogen, C. Fibrinogen, D. Heparin
A: Antithrombin III
Answer: A: Antithrombin III 
Explanation: A i.e. Antithrombin III