Select the true statements about protein secondary structure.
A. In a β‑pleated sheet, the side chains extend above and below the sheet.
B. The secondary level of protein structure refers to the spatial arrangements of short segments of the protein.
C. Disulfide bonds stabilize secondary structure. In an α‑helix, the side chains are located inside the helix.
D. The α‑helix is held together by hydrogen bonds between the amide N − H and C = O groups.