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
Serum is: 
Select and explain. 
A. Plasma-clotting factors, B. Plasma-fibrin, C. Blood-RBC, D. Blood-all cellular elements
B: Plasma-fibrin
Answer: B: Plasma-fibrin 
Explanation: None