Answer:
For a gas held at constant temperature, we can apply Boyle's law, which states that the product between the gas pressure and its volume is constant:
where
P is the pressure
V is the volume
As we see from the equation, P and V are inversely proportional to each other: this means that when the volume is decreased, the pressure increases, and vice-versa. The reason for that is that when the volume is decreased, the gas is compressed, so the molecules of the gas come closer to each other, so they collide more frequently with the wall of the container, exerting therefore a greater pressure.