In considering protein secondary structure which of the following is INCORRECT?

A) An α helix repeats after 18 residues and has 3.6 residues per turn.
B) A network of main-chain hydrogen bonds connect β strands in a β sheet.
C) The most common structures are the α helix and the β sheet.
D) The 310 helix is right-handed and often contains proline residues.
E) The β strands can be in either parallel or antiparallel configuration.