What is used in passive solar heating systems? 

A)  a concrete wall 
B)  a paper wall 
C)  a natural gas furnace 
D)  photovoltaic cells

Passive Solar Heating

Utilizing the sun's energy for heating spaces without the use of mechanical systems by strategically placing windows and selecting materials that absorb and distribute heat.

Concrete Wall

A robust and durable barrier made of concrete blocks or poured concrete, commonly used for structural and protective purposes.

Photovoltaic Cells

Devices that convert light energy directly into electrical energy through the photovoltaic effect, commonly used in solar panels.