Two vessels separately contain two ideal gases $A$ and $B$ at the same temperature, the pressure of a being twice that of $B$. Under such conditions, the density of $\mathrm{A}$ is found to be $1.5$ times the density if $\mathrm{B}$. The ratio of molecular weight of $\mathrm{A}$ and $\mathrm{B}$ is: (1) $\frac{1}{2}$ (2) $\frac{2}{3}$ (3) $\frac{3}{4}$ (4) 2