This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Human identification, Subject name: Forensic Medicine
The racemization of which of the following in is used for age estimation from teeth 
Select and explain. 
A. Alanine, B. Aspaic acid, C. Glycine, D. Cysteine
B: Aspaic acid
Answer: B: Aspaic acid 
Explanation: Aspaic acid racemization: Of all amino acids, aspaic acid has one of the fastest racemization rates and most commonly used for age estimation. During the course of aging, L-forms of amino acids are transformed by racemization to D-forms. Thus, the extent of racemization of amino acids may be used to estimate the age . The analysis has been done on both tooth enamel and crown dentin with analysis of dentin giving more accurate age estimation than enamel.