Answer the following question given this paragraph: 

To this point, we have examined the relationships between any two of the variables of , , and , while the third variable is held constant. However, situations arise where all three variables change. The combined gas law expresses the relationship between the pressure, volume, and absolute temperature of a fixed amount of gas. For a combined gas law problem, only the amount of gas is held constant.


Q: The combined gas law involves three properties of a gas - volume, absolute temperature, and what?


A:
pressure