A piston traps air inside a cylinder. The air pressure is $p$ and the air column has a length of $20\ \text{cm}$. The piston is then pulled outwards until the air column is $40\ \text{cm}$ longer than before. The temperature of the air stays constant. What is the air pressure now?
- A$\frac{p}{2}$
- B$\frac{p}{3}$
- C$2p$
- D$3p$