In 1620,about half the USA was covered by forests .Today the forests have almost gone .A lot of good land has gone with them ,leaving only sand .China doesn't want to copy the USA's example.We're planting more and more tress .We've built the "Great Green Wall "of tress across northern pat of our country .The "Great Green Wall"is 7,000 kilometers long ,and between 400 and 1,700 kilometers wide .It will stop the wind from blowing the earth away .It will stop the sand from moving towards the rich farmland in the south .More "Great Green Wall "are needed .Trees must be grown all over the world. "Great Green Wall "will make the world better . ,. The "Great Green Wall"In China is   _  long
7,000 kilometers 1,700 kilometers 7,000 meters 400 kilometers