What has the highest net energy ratio for space heating? 

A)  passive solar 
B)  oil 
C)  active solar 
D)  electric resistance heating

Passive solar heating systems use the sun's energy to naturally heat a space, without the need for mechanical or electrical systems. Because they do not require external energy inputs, they have a high net energy ratio compared to other heating sources. Active solar heating also uses solar energy, but requires mechanical systems to circulate the heated air or liquid, which lowers its net energy ratio. Oil and electric resistance heating both have low net energy ratios due to the high energy inputs required to extract or produce the fuel or electricity.