Aheating system musty maintain the interior of a building at 20 degree c during a period when the outside air temperature is 5 degree c and the heat transfer from the building through its roof and walls is 3 x 10^8 kj. for this duty heat pumps are under consideration that would operate between the dwelling and (a) the ground at 15 degree c . (b) a pond at 10 degree c. (c) the outside air at 5 degree c.