Consider the following statements about Langmuir isotherm:

$\left(\mathrm{i}\right)$ The free gas and adsorbed gas are in dynamic equilibrium.

$\left(\mathrm{ii}\right)$ All adsorption sites are equivalent.

$\left(\mathrm{iii}\right)$The initially adsorbed layer can act as a substrate for further adsorption.

$\left(\mathrm{iv}\right)$The ability of a molecule to get adsorbed at a given site is independent of the occupation of neighbouring sites.

The correct statements are: