Final answer:
Curve B most likely represents temperature, as explained by the context of the triple point and state changes of water under constant pressure conditions.
Step-by-step explanation:
The curve B most likely represents temperature. This is because in the context provided, the graphs relate to changes in states of matter under different conditions of temperature and pressure. Specifically, the point B in Figure 10.31 is described as the point where all three phases of water coexist in equilibrium, which is known as the triple point. The triple point occurs at a specific temperature and pressure, and since curve B intersects at this point, it suggests that the variable being held constant along that curve is temperature. Therefore, when the substance changes from solid to liquid to gas upon heating at a constant pressure, the temperature at those transitions—represented by the normal melting point and normal boiling point—help confirm that curve B is showing temperature changes.
Curve B most likely represents Volume in the graph. In the context of the question, the graph plots the absolute pressure (P) of a gas as a function of its temperature (T). Since Volume is one of the properties that can be determined from the graph, it can be inferred that curve B represents the Volume of the gas at different temperatures.