In the following compound,the favourable site/s for protonation is/are:
(Image description: Adenine structure with labels $a$ on the exocyclic $-NH_2$ group,$b$ on the pyrimidine ring nitrogen,$c$ on the other pyrimidine ring nitrogen,$d$ on the imidazole ring nitrogen,and $e$ on the imidazole ring $-NH-$ group.)

• A
  $a$ and $e$
• B
  $b, c$ and $d$
• C
  $a$ and $d$
• D
  $a$