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: Medicine
The blood in the vessels normally does not clot because - 
Select and explain. 
A. Vitamin K antagonists are present in plasma, B. Throbin has a positive feedback on plasminogel, C. Sodium citrate in plasma chelates calcium ions, D. Vascular endothelium is smooth and coated with glycocalyx
D: Vascular endothelium is smooth and coated with glycocalyx
Answer: D: Vascular endothelium is smooth and coated with glycocalyx 
Explanation: None