At $323 \mathrm{~K}$, the vapour pressure (in $\mathrm{mm}$ $\mathrm{Hg}$ ) of a methanol-ethanol solution is represented as:
$$
P=120 X+140
$$
where $X$ is the mole fraction of methanol in liquid solution at equilibrium. Then the value of $P_{\mathrm{E} \text { ton }}^{\circ}$ is
(a) $120 \mathrm{~mm}$
(b) $140 \mathrm{~mm}$
(c) $260 \mathrm{~mm}$
(d) $20 \mathrm{~mm}$