Which are the most favorable conditions for the liquefaction of a gas?

Critical temperatures for $A$,$B$,$C$ and $D$ gases are $25\,^{\circ}C$,$10\,^{\circ}C$,$-80\,^{\circ}C$ and $15\,^{\circ}C$ respectively. Which gas will be liquefied more easily?

What happens when the temperature of $CO_2$ at its critical temperature of $30.98 \, ^\circ C$ is increased or decreased?

Gases possess characteristic critical temperature which depends upon the magnitude of intermolecular forces between the gas particles. Critical temperatures of ammonia and carbon dioxide are $405.5 \ K$ and $304.10 \ K$ respectively. Which of these gases will liquefy first when you start cooling from $500 \ K$ to their critical temperature?

$A$ real gas cannot be liquefied in which of the following conditions?

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?