Options :

1. Hydrophilic sugar-phosphate groups are found on the exterior of the helix where interaction with water occurs

2. Hydrophobic bases are present in the interior of the helix, each base-pair is stabilized by the same number of hydrogen bonds

3. covalent base stacking interactions may take place between neighbouring bases within the same strand in the helix

4. non-covalent N-glycosidic bonds may form between nitrogenous bases in opposite strands in the helix