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: Pathology
Strength of collagen is due to: 
Select and explain. 
A. Hydroxy glycine, B. Glycine, C. Praline, D. Hydroxyproline
D: Hydroxyproline
Answer: D: Hydroxyproline 
Explanation: None