18. Thr and/or Leu residues tend to disrupt an a helix when they occur next to each other in a protein because: (A) of electrostatic repulsion between the Thr and/or Leu side chains. (B)of steric hindrance between the bulky Thr and/or Leu side chains.5 ㆍ(C) both amino acids are highly hydrophobic. (D) the R group of neither amino acid can form a hydrogen bond. (E) of the possible covalent interactions between the Thr and/or Leu side chains.