In 1620, about half the USA was (11) forests. Today the forests have almost gone. A lot of good land has (12) them, leaving only sand. China doesn’t want to copy the USA’s example. We’re planting more and more trees. We’ve built the "Great Green Wall" of trees across (13) part of our country. The "Great Green Wall" is 7,000 kilometers long, and between 400 and 1,700 kilometers (14) . It will stop the wind from blowing the earth away. It will stop the sand from moving towards the (15) farmland in the south.