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: Medicine
[{'content': 'Most common trisomy among following is - \nSelect and explain. \nA. 18, B. 21, C. 13, D. 5', 'role': 'user'}, {'content': "Answer: B \nExplanation: The most common abnormalities of chromosome number are 'trisomies' -Nelson\nThe most frequent and best known trisomy in humans is trisomy 21 or Down's syndrome - Nelson", 'role': 'assistant'}]