In the sign convention where energy transferred into a system as work is taken to be positive, which statement correctly identifies the sign of the work term W when an ideal gas expands against a constant external pressure?
a) W is positive because the gas does work on the surroundings during expansion.
b) W is zero because the pressure is constant, so the positive and negative contributions to work cancel.
c) W is negative because the gas does work on the surroundings during expansion.
d) W is positive because expansion increases volume and both pressure and volume are positive in W=PΔV."