阅读理解

    Humans and animals need oxygen to exist. Without it, most living things will die. But where does oxygen come from'? Trees. Trees can take in much carbon dioxide which may be bad for our environment and produce oxygen for us to breathe.

    All people know that trees are important to us. Scientists believe humans cannot live if there are no trees on the Earth.

    Trees are a major source(来源) of food. Apples, oranges, lemons all come from trees.  Humans and animals knew how to get food from trees many years ago.

    In fact, many kinds of important medicine come from trees too. Some special things on trees can be used to produce medicine. Also, trees can make the world beautiful and relaxing. They help protect the soil.