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 primary structure of a protein refers to 
Select and explain. 
A. Linear structure and order of the amino acids present, B. Regular confirmational forms of a protein, C. Complete three-dimensional structure of the polypeptide units of a given protein, D. Subunit structure of the protein
A: Linear structure and order of the amino acids present
Answer: A: Linear structure and order of the amino acids present 
Explanation: The linear sequence of amino acid residues and location of disulphide bridges, if any, in a polypeptide constitute its primary structure.