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
Bonding agents have hydrophilic and hydrophobic groups. The hydrophobic group binds which structure? 
Select and explain. 
A. Calcium in enamel, B. Hydroxyapatite group in hard tissue, C. Resin in restoration, D. Collagen in dentin
C: Resin in restoration
Answer: C: Resin in restoration 
Explanation: Ideally, a dentin bonding agent should have both hydrophilic and hydrophobic ends. The hydrophilic end displaces the dentinal fluid to wet the surface. The hydrophobic end bonds to the composite resin.
Bonding: The forces or energies between atoms or molecules at an interface that hold two phases together.
Mechanism of bonding:
Dentin adhesive molecule has a bifunctional structure:
M-R-X
Where,
M is the double bond of methacrylate which copolymerizes with composite resin.
R is the spacer which makes the molecule large.
X is a functional group for bonding, which bonds to an inorganic or organic portion of dentin.
Ref: Textbook of Operative Dentistry, Nisha and Amit Garg, 3rd edition, Pg no:239