This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
Role of Vitamin K for activation in clotting cycle 
Select and explain. 
A. Carboxylation, B. Hydroxylation, C. Oxidation, D. Reduction
D: Reduction
Answer: D: Reduction 
Explanation: Vitamin K is an essential factor to a liver carboxylase that adds a carboxyl group to glutamic acid residues on five of the impoant clotting factors: prothrombin, Factor VII, Factor IX, Factor X, and protein C. Upon adding the carboxyl group to glutamic acid residues on the immature clotting factors, vitamin K is oxidized and becomes inactive. Another enzyme, vitamin K epoxide reductase complex 1 (VKOR c1), reduces vitamin K back to its active form.Ref: Guyton and Hall textbook of medical physiology 13th edition. Page:490