Proteins have a 'backbone' of repeating elements, based on the similar composition of amino acids. The repeating elements of a polypeptide backbone...

1. Proteins have a 'backbone' of repeating elements, based on the similar composition of amino acids. The repeating elements of a polypeptide backbone are: a. S-C-C-S-C-C... b. N-C-C-N-C-C... c. N-C-O-N-C-O... d. C-O-O-C-O-O... e. C-P-C-C-P-C... I dont get this question? Help me