Consider the following statements about Langmuir isotherm.
$I.$ The free gas and adsorbed gas are in dynamic equilibrium.
$II.$ All adsorption sites are equivalent.
$III.$ The initially adsorbed layer can act as a substrate for further adsorption.
$IV.$ The ability of a molecule to get adsorbed at a given site is independent of the occupation of neighboring sites.
The correct statements are

• A
  $I, II, III$ and $IV$
• B
  $I, II$ and $IV$
• C
  $I, III,$ and $IV$
• D
  $I, II,$ and $III$