Carbonic acid is a mixture of

a compound of the elements hydrogen, carbon, and oxygen. (also carbonic acid is a compound not a mixture ^-^)

Explanation:

hope this helps dude

Obviously from the graph, the relationship is linear. Charle's laws also say that so that one can be eliminated. Also using the points on the graph, abolute 0 is -273 and liquefication occurs around -155. That sets the answer to Condensation occurs at 0