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 of the following amino acids in a protein commonly acts as a potential O-Glycosylation site for attachment of an oligosaccharide unit - 
Select and explain. 
A. Glutamine, B. Cysteine, C. Serine, D. Asparagine
C: Serine
Answer: C: Serine 
Explanation: O-glycosylation:- Addition of oligosaccharide to hydroxyl chain of serine or threonine.
N-glycosylation:- Addition of oligosaccharide to the amide nitrogen of asparagine.