The variation of vapour pressure of different liquids with temperature is shown in the figure. At high altitude, atmospheric pressure is low (say $60 \mathrm{mm} \mathrm{Hg}$). At what temperature liquid $\mathrm{D}$ boils?     

The variation of vapour pressure of different liquids with temperature is shown in the figure. Pressure cooker is used for cooking food at hill station. Explain in terms of vapour pressure why is it so?

 Isotherms of carbon dioxide gas are shown in the figure. Mark a path for changing gas into liquid such that only one phase (i.e., either a gas or a liquid) exists at any time during the change. Explain how the temperature, volume and pressure should be changed to carry out the change.