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
Coenzyme used in Kreb's cycle ? 
Select and explain. 
A. NAD, B. NADP, C. NADPH, D. NADH
A: NAD
Answer: A: NAD 
Explanation: Niacin is used as coenzyme nicotinamide adenine dinucleotide (NAD') for transfer of hydrogen.