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
In quaternary structure, subunits are linked by: 
Select and explain. 
A. Peptide bonds, B. Disulphide bonds, C. Covalent bonds, D. Non-covalent bonds
D: Non-covalent bonds
Answer: D: Non-covalent bonds 
Explanation: The forces that keep the quaternary structure are hydrogen bonds, electrostatic bonds, hydrophobic bonds and van der Waals forces.
Reference: Vasudevan 7th  ed, pg 38