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 the precursor amino acid for selenocysteine 
Select and explain. 
A. Cysteine, B. Cystine, C. Serine, D. Glycine
C: Serine
Answer: C: Serine 
Explanation: None