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 conversion of prothrombin to thrombin, the factor to participate along with calcium is: 
Select and explain. 
A. Labile factor, B. Stable factor, C. Fibrin, D. Thromboplastin
A: Labile factor
Answer: A: Labile factor 
Explanation: None