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: Pharmacology
Anticoagulant not used in vitro is : 
Select and explain. 
A. Heparin, B. Warfarin, C. Oxalate, D. Citrate
B: Warfarin
Answer: B: Warfarin 
Explanation: None