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: Pathology
Conversion of prothrombin to thrombin requires: 
Select and explain. 
A. V only, B. V and Ca++, C. XII, D. X and Ca++
D: X and Ca++
Answer: D: X and Ca++ 
Explanation: None