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
Which among the following is the structure of hemoglobin? 
Select and explain. 
A. Monomer, B. Homodimer, C. Heterodimer, D. Tetramer
D: Tetramer
Answer: D: Tetramer 
Explanation: Hemoglobins are tetramers composed of pairs of two different polypeptide subunits.
Ref: Harper’s illustrated biochemistry. 30th edition page no: 54