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
Which is must for prothrombin time? 
Select and explain. 
A. Thromboplastin, B. Prothrombin, C. Fibrinogen, D. Fibrin
A: Thromboplastin
Answer: A: Thromboplastin 
Explanation: Prothrombin time test - Time needed for plasma to clot after addition of tissue thromboplastin and Ca2+ ions.