A horizontal cylinder with cross-sectional area $A$ is equipped with a frictionless piston and contains air at pressure $P$. It is placed in hot water, causing the length of the air column to rise from $x$ to $y$. The pressure $P$ stays constant. Which equation shows the work done by the trapped air during this process?
- A$PAy$
- B$-PAy$
- C$PA(y - x)$
- D$-PA(y - x)$