Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R. Assertion A: Treatment of immunoglobulin IgG with β-mercaptoethanol gives rise to two heavy chains and two light chains peptides on SDS-PAGE. Reason R: β-mercaptoethanol breaks peptide bond at the site of disulphide linkage. In the light of the above statements, choose the most appropriate answer from the options given below: