问题描述：
谁给这篇英语短文取个题目
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 trees． We've built the " Great Green Wall" of trees across northern part of our country．The Great Green Wall is 7,000 kilometres long,and between 400 and 1,700 kilometres 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 Walls" are needed． Trees must be grown all over the world． Great Green Walls will make the world better．
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 trees． We've built the " Great Green Wall" of trees across northern part of our country．The Great Green Wall is 7,000 kilometres long,and between 400 and 1,700 kilometres 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 Walls" are needed． Trees must be grown all over the world． Great Green Walls will make the world better．
问题解答：
我来补答展开全文阅读