The van der Waals equation for one mole of a real gas can be...

The van der Waals equation for one mole of a real gas can be written as \left(\mathrm{P}+\mathrm{a} / \mathrm{V}^2\right)(\mathrm{V}-\mathrm{b})=\mathrm{RT}$. For the gases$\mathrm{H}_2, \mathrm{NH}_3$, and$\mathrm{CH}_4$, the value of ' a ' \left(\right.$ bar $\mathrm{L}^{-2} \mathrm{mol}^{-2}$ ) are $0.2453,4.170$ and 2.253 respectively. Which of the following can be inferred from the 'a' values?