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
The biological anticoagulant is: 
Select and explain. 
A. EDTA, B. Sodium citrate, C. Hirudine, D. Double oxalate mixture
C: Hirudine
Answer: C: Hirudine 
Explanation: None