A piston with cross-sectional area $A$ confines gas inside a cylinder. The piston is not frictionless. When the gas is heated, it expands and drives the piston back by distance $r$, moving it from position P to position Q. The gas column then has length $s$. Which expression gives the work done by the gas against the atmosphere during this expansion?
- A$(\text{atmospheric pressure}) \times A r$
- B$(\text{atmospheric pressure}) \times A s$
- C$(\text{pressure inside the gas}) \times A r$
- D$(\text{pressure inside the gas}) \times A s$