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
[{'content': 'Which is 21stamino acid \nSelect and explain. \nA. Alanine, B. Cystine, C. Arginine, D. Seleno cysteine', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Selenocysteine is now considered as 21st amino acid.', 'role': 'assistant'}]