Critical temperature of a gas

The critical temperature of a gas 

(a) is the temperature above which a gas cannot be liquefied

(b) is the temperature below which it requires a lower pressure to liquefy a gas

(c) varies with pressure

(d) is the temperature at which a gas exists simultaneously in the gaseous and liquid states at atmospheric pressure

(e) is the temperature at which oxygen is liquefied by a pressure of 50 bar

(a) true (b) true (c) false (d) false (e) true