What is the Andrews isotherm graph used for?

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

The gas with the highest critical temperature is

Under which of the following conditions can a gas be liquefied?

The number of statements,which are correct with respect to the compression of carbon dioxide from point $(a)$ in the Andrews isotherm from the following is $..........$.
$A.$ Carbon dioxide remains as a gas up to point $(b)$
$B.$ Liquid carbon dioxide appears at point $(c)$
$C.$ Liquid and gaseous carbon dioxide coexist between points $(b)$ and $(c)$
$D.$ As the volume decreases from $(b)$ to $(c)$,the amount of liquid decreases

However great the pressure,a gas cannot be liquefied above its