6. Planarity of the peptide bond means that rotation is allowed about the bond linking___________the and the carbon of the peptide bond, and also about the bondlinking the___________ to the adjacent α-carbon.
(A) a-carbon, carbony1 carbon
(B) B-carbon, carbonyl carbon
(C) carbonyl carbon, nitrogen of the peptide bond
(D) a-carbon, nitrogen of the peptide bond
(E) none are true