article: What will the house of the future look like? Architects   believe that there are all possibilities. The only thing for sure is that the house will be as green as possible.
  The tree house
  Many architects in the world would like to build a "tree house." Like a leaf, the surface   of the house collects sunlight during the day. The energy can be used to heat water, produce electricity, and even create fresh air for the home. The "root" of the house is deep under the ground.
  The lizard   house
  Like a lizard, changing color with the weather is the most important design of a lizard house. When it's in the bright sun, the cover of the house will turn dark to protect it from strong heat. During dark days, it turns white and takes in as much light and heat as possible to produce energy.
  Meals at home
  It has gardens on the outside wall of the house. People can plant tomatoes, carrots and green tea on them. So every day in the morning, you just need to walk outside and collect your meals.
  Learning from the past
  Looking to the future isn't the only way to be green. Sometimes, ancient techniques   can also help cut down energy use. For example, a chimney can be a useful air conditioner because it is easier for hot air to flow out at the chimney.
question: What's the passage mainly about?
options: Future life Green house of the future Different houses Energy saving in the future