Liquefaction of a gas can occur $....................$

Under which of the following conditions can a gas be liquefied?

Why does the boundary between the liquid phase and the gaseous phase disappear upon heating a liquid up to the critical temperature in a closed vessel? In this situation,what will be the state of the substance?

Amongst the following statements,the correct one is

Critical temperature for carbon dioxide and methane are $31.1^{\circ} C$ and $-81.9^{\circ} C$ respectively. Which of these has stronger intermolecular forces and why?

At what pressure does $SO_2$ gas liquefy at room temperature?