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
Which is 21stamino acid 
Select and explain. 
A. Alanine, B. Cystine, C. Arginine, D. Seleno cysteine
D: Seleno cysteine
Answer: D: Seleno cysteine 
Explanation: Selenocysteine is now considered as 21st amino acid.