A single machine is first used as a refrigerator and then as a heat pump while operating between the same cold and hot reservoirs. Which statement is correct?
(a) COPHP is larger because the work input is smaller in heat-pump mode for the same temperature limits.
(b) COPHP equals COPref because both depend only on the reservoir temperatures.
(c) COPHP is larger because the heat delivered to the hot reservoir includes both the absorbed heat from the cold side and the work input.
(d) COPref is larger because removing heat from the cold reservoir is easier than delivering it to the hot reservoir.