In the series $HCl$,$HBr$ and $HI$,the boiling point increases in the order $HCl < HBr < HI$. This is due to

• A
  $HI$ is the strongest acid among the series
• B
  $HI$ is the strongest reducing agent among the series
• C
  higher van der Waals' forces of attraction in $HI$
• D
  intermolecular $H$-bonding in $HI$