Thr And/or Leu Residues Tend to Disrupt An $\alpha$ Helix When They Occur Next to Each Other in a When

Multiple Choice

Thr and/or Leu residues tend to disrupt an $\alpha$ helix when they occur next to each other in a protein because:

A) an amino acids like Thr is highly hydrophobic.
B) covalent interactions may occur between the Thr side chains.
C) electrostatic repulsion occurs between the Thr side chains.
D) steric hindrance occurs between the bulky Thr side chains.
E) the R group of Thr can form a hydrogen bond.