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

What is the surface tension at the critical temperature?

What is meant by the vapor of a substance like $CO_2$?

The critical temperatures for carbon dioxide $(CO_2)$ and methane $(CH_4)$ are $31.1 \, ^\circ C$ and $-81.9 \, ^\circ C$ respectively. Which of the following statements is correct regarding their liquefaction?

$A, B, C$ and $D$ are four different gases with critical temperatures $304.1, 154.3, 405.5$ and $126.0 \ K$ respectively. While cooling the gas,which gets liquefied first?

Isotherms of carbon dioxide at various temperatures are represented in the figure. Answer the following questions based on this figure.
$(i)$ In which state will $CO_2$ exist between the points $a$ and $b$ at temperature $T_1$?
$(ii)$ At what point will $CO_2$ start liquefying when temperature is $T_1$?
$(iii)$ At what point will $CO_2$ be completely liquefied when temperature is $T_2$?
$(iv)$ Will condensation take place when the temperature is $T_3$?
$(v)$ What portion of the isotherm at $T_1$ represents liquid and gaseous $CO_2$ at equilibrium?