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: Dental
Which of the following is not a natural composite? 
Select and explain. 
A. Enamel, B. Dentin, C. Hydroxyapatite crystals, D. None
C: Hydroxyapatite crystals
Answer: C: Hydroxyapatite crystals 
Explanation: Composite literally meaning “made up of several parts or elements.”
Enamel and dentine acts as composite structure. 
Hydroxyapatite crystals acts as ﬁller.