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
The function of vitamin K is: 
Select and explain. 
A. Thrombin formation, B. Prothrombin formation, C. Fibrin formation, D. Thromboplastin formation
B: Prothrombin formation
Answer: B: Prothrombin formation 
Explanation: None