There are some places in the world where great cities once stood. There were trees, gardens and grass. But now these cities have gone and the places are like deserts. Scientists began to study this land to find out what happened. Many of them believed the land died when the trees were cut down. Trees help other plants grow and help hold the soil in place. They keep the soil from blowing away. The leaves of trees on the ground keep rainwater from running off. Trees near farms protect growing plants from strong winds. Once there were hundreds of farms in the southwestern United States. Then the farms turned into fields of dry dust. Trees might have saved these farms. But few trees grow in the dry southwest now. Now we try to save trees. In many places, when a tree is cut down , a new tree is planted in its place . We do not want the land to die. The main idea of this passage is   _   .
that farms are better than cities how farms turned into deserts that trees are important to the land how people save trees