Which of the following contributes to the stability of the DNA double helix?

Which of the following contributes to the stability of the DNA double helix?
 
  A) hydrophobic interactions between aromatic bases at the center of the double helix
  B) hydrogen bonding between the phosphate and sugar groups in the backbone of the double helix
  C) covalent bonding between complementary purine and pyrimidine bases.
  D) ionic bonds between the negatively charged phosphate groups and the positively charged pyrimidine bases.
  E) hydrophobic interactions between aromatic bases at the center of the double helix and ionic bonds between the negatively charged phosphate groups and the positively charged pyrimidine bases