Which of the following statements about α helices and β−pleated sheets in protein secondary structures are correct?
I. Alpha helices are stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of an amino acid four residues earlier.
II. Beta-pleated sheets are formed by hydrogen bonding between the side chains of amino acids in adjacent beta strands.
III. Alpha helices are typically right-handed spirals, while beta-pleated sheets can be arranged in parallel or antiparallel orientations.
IV. The beta-pleated sheet structure involves interactions between backbone amide hydrogens and carbonyl oxygens of adjacent beta strands.

• A. I, II, and III
• B. I, II, and IV
• C. I, III, and IV
• D. I, II, III, and IV

Answer: (C)