This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: vitamins, Subject name: Biochemistry
[{'content': 'Which vitamin is required for transfer of 1-carbon unit? \nSelect and explain. \nA. Vitamin A, B. Folic acid, C. Vitamin B12, D. Niacin', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Tetrahydrofolate can carry one-carbon fragments attached to N-5 (formyl, formimino, or methyl groups), N-10 (formyl) or bridging N-5-N-10 (methylene or methenyl groups). 5-Formyl-tetrahydrofolate is more stable than folate and is therefore used pharmaceutically (known as folinic acid), and the synthetic (racemic) compound (leucovorin).The major point of entry for one-carbon fragments into substituted folates is methylene-tetrahydrofolate.', 'role': 'assistant'}]