In terms of $DNA$ and $RNA$ structure,what is a nucleotide?

• A
  $A$ nucleotide is a heterocyclic base.
• B
  $A$ nucleotide is a sugar molecule covalently bonded to a heterocyclic base.
• C
  $A$ nucleotide is a sugar molecule bonded to a phosphate group and a heterocyclic base.
• D
  $A$ nucleotide is a heterocyclic base bonded to a phosphate group.