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: Surgery
Pre-operative investigation recommended before surgical procedures in a patient on warfarin therapy is 
Select and explain. 
A. Partial thromboplastin time, B. Clotting time, C. International Normalized Ratio, D. Differential count
C: International Normalized Ratio
Answer: C: International Normalized Ratio 
Explanation: None