Why do complementary nucleotides across the double-stranded dna bond together using hydrogen bonds rather than covalent bonds?

a) covalent bonds would be too strong to allow the double helix to easily separate and make templates available for replication and transcription.

b) covalent bonds are energetically too expensive to produce.

c) covalent bonds would not allow bases to pair according to chargaff's rules.

d) covalent bonding across the double helix would make the dna less stable than hydrogen bonding across the double helix.

e) there are no enzymes that are capable of forming covalent bonds across the double helix.