What are the critical constants?

Two different gases $A$ and $B$ are filled in separate containers of equal capacity under the same conditions of temperature and pressure. On increasing the pressure slightly,gas $A$ liquefies,but gas $B$ does not liquefy even on applying high pressure until it is cooled. Explain this phenomenon.

An ideal gas cannot be liquefied because ........

$A$ gas can be liquefied at temperature $T$ and pressure $p$ provided:

Assertion : Gases do not liquefy above their critical temperature,even on applying high pressure.
Reason : Above critical temperature,the molecular speed is high and intermolecular attractions cannot hold the molecules together because they escape because of high speed.

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?