A protein's secondary structure arises due to interactions between amino acids. Two common secondary structures found in many proteins are α helices and β sheets. Classify the statements as pertaining to α helices or β sheets.

a. flat, zig-zag-like structure
b. two adjacent polypeptide chains hold each other in place with hydrogen bonds
c. coil-like structure around a central axis
d. form long, thin structures important for transmembrane proteins