You have a gas at 25 C confined to a cylinder with a movable piston. Which of the following actions would double the gas pressure? (a) Lifting up on the piston to double the volume while keeping the temperature constant (b) Heating the gas so that its temperature rises from 25 C to 50 C, while keeping the volume constant (c) Pushing down on the piston to halve the volume while keeping the temperature constant.