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
The 40 nm gap in between the tropocollagen molecule in collagen which serve as the site of bone formation is occupied by which of the following- 
Select and explain. 
A. Carbohydrate, B. Ligand moiety, C. Calcium, D. All
C: Calcium
Answer: C: Calcium 
Explanation: C i.e. Calcium40 nm gap between the typrocollagen molecues serve as nucleation site for deposition of hydroxyapatite I ca5(p04)3(19H),1 with some phosphate] crystals in boneQ.