Protein secondary structure elements such as α helices and β sheets constitute the major regular folding patterns in proteins. With regard to these elements, …

A) hydrogen-bonding between the amino acid side chains defines the type of secondary structure.
B) a certain short amino acid sequence always adopts the same secondary structure.
C) only a few specific amino acid sequences can adopt these repetitive structures.
D) the folding patterns result from hydrogen-bonding between the N-H and C=O groups in the polypeptide backbone.
E) All of the above.