This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Basics of amino acids, Subject name: Biochemistry
Amino acid with double chiral carbon- 
Select and explain. 
A. Tyrosine, B. Threonine, C. Tryptophan, D. Phenylalanine
B: Threonine
Answer: B: Threonine 
Explanation: All amino acids have 1 assymetric or chiral C (means this C has all 4 valencies occupied by different atoms or group of atoms) EXCEPTIONS: * 0 - Glycine (So it lacks chilarity. Hence glycine won't show any optical activity) * 2 - Isoleucine, Threonine (double chiral carbons)