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 is not involved in blood clotting? 
Select and explain. 
A. Calcium, B. Prothrombin, C. Fibrinogen, D. Fe+2
D: Fe+2
Answer: D: Fe+2 
Explanation: None