At what point does surface tension vanish?

Four gas cylinders containing $He$,$N_2$,$CO_2$ and $NH_3$ gases separately are gradually cooled from a temperature of $500 \ K$. Which gas will liquefy first?
(Given $T_C$ in $K - He: 5.3, N_2: 126, CO_2: 304.1$ and $NH_3: 405.5$)

Which of the following statements is incorrect?

Which statement$(s)$ is/are applicable above critical temperature?

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

According to Andrews isothermals,at what temperature does carbon dioxide gas start to condense at $73 \ atm$ (in $^{\circ} C$)?