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
Most common trisomy among following is - 
Select and explain. 
A. 18, B. 21, C. 13, D. 5
B: 21
Answer: B: 21 
Explanation: The most common abnormalities of chromosome number are 'trisomies' -Nelson
The most frequent and best known trisomy in humans is trisomy 21 or Down's syndrome - Nelson