Arrange the following gases in increasing order of van der Waals...

The van der Waals' constant $′ a ′$is directly proportional to the intermolecular force of attraction; hence, the gas with greater value of $′ a ′$ can be most easily liquefied.The values of Vander Waal's constant $' a '$ for the given gases are:
Gases Vander Waal's Constant $' a '$
A. $Ar$ $1 . 35{L}^{2}$ atm ${mol}^{2}$
B. ${CH}_{4}$ $2 . 25{L}^{2}$ atm ${mol}^{2}$
C. ${H}_{2}O$ $5 . 536{L}^{2}$ atm ${mol}^{2}$
D. ${C}_{6}{H}_{6}$ $18 . 57{L}^{2}$ atm ${mol}^{2}$
$∴$Increasing order of $' a '$ for the given gases
$A < B < C < D$