Which are sources for thermal energy for a heat pump? Check all that apply.

air

water

sunlight

ground

fire

I would say ground is the main one because I know that some geotechnical consultants install heat pumps underground to harness geothermal energy but not the geothermal of the hot water but of simply by being within the earth. Also, water could apply as well since true geothermal energy presumably with heat pumps also comes from hot water circulating deep within the earth's crust such as the source for geysers. The volcano Izalco gives off hot water through the cracks in its' surface so could probably be used to power a heat pump also.