article: Millions of people around the world cook their food on fire every day. People must spend lots of money on cooking _ . However, there is a much easier and cheaper way to cook food using energy from the sun.
  Sun-cookers have been used for centuries. A Swiss scientist made the first sun-cooker in 1777. Today, people are using sun-cookers in many countries around the world. People use them to cook food and to heat drinking water.
  There are three kinds of sun-cookers. The first is a box cooker. It is designed with a special wall that collects sun-shine into the box. A box cooker is good for slow cooking of a lot of food.
  The second kind of sun-cooker is a panel cooker. It has several flat walls that concentrate the sun-shine on the food. People can build panel cookers quickly. They do not cost much. In Kenya, for example, panel cookers cost just two dollars.
  The third kind of sun-cooker is a parabolic cooker. It has rounded walls that concentrate sunlight into the bottom of the cooker. Food cooks quickly in parabolic cookers. However, these cookers are hard to make. They must be moved again and again to follow the sun. Parabolic cookers can also cause burns and eye injuries if they are not used correctly.
  You can make sun-cookers from boxes or heavy paper. They will not catch fire. Paper burns at 232degC. A sun-cooker never gets hotter than that. Sun-cookers cook food at low temperatures over long periods of time. This allows people to cook food and do other things at the same time.
question: Which kind of sun-cooker costs very little?
options: a box cooker a panel cooker a parabolic cooker fire cooker