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: Biochemistry
The cofactor involved in sulphur containing amino acid metabolism is which of the following? 
Select and explain. 
A. Folic acid, B. Biotin, C. Vitamin B1, D. Vitamin B12
D: Vitamin B12
Answer: D: Vitamin B12 
Explanation: While both folic acid and vitamin B12 are involved in sulphur containing amino acid metabolism, vitamin B12 acts as cofactor and folate acts as substrate.