Study the following diagram showing the structure of proinsulin and select the option that correctly identifies the labels X, Y and Z.

	X	Y	Z
1.	A peptide	B peptide	C peptide
2.	C peptide	B peptide	A peptide
3.	B peptide	C peptide	A peptide
4.	C peptide	A peptide	B peptide

X is C peptide; Y is A peptide and Z is B peptide. 

A peptide has an intrachain disulphide bond. So, Y can be identified as A chain. There are two interchain disulphide bonds between A chain and B chain. So, Z can be identified as B chain. X is the C chain that is removed during the maturation of proinsulin to insulin.