Benzene and toluene form an ideal solution. At a certain temperature the vapour pressures of pure benzene and pure toluene are $120\mathrm{torr}$ and $40\mathrm{torr}$. A liquid mixture is prepared from $78\mathrm{g}$ of benzene and $138\mathrm{g}$ of toluene (molar masses: benzene $78$, toluene $92\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$). The total vapour pressure of the mixture at this temperature, in torr, is

1. A88.0
2. B72.0
3. C68.9
4. D80.0