阅读理解：阅读下列短文，从每题A、B、C、D 四个选项中，选出一个最佳答案。
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 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 Walls" are needed． Trees must be grown all over the world． Great Green Walls will make the world better．
根据短文内容，选择正确答案
（ ）1．In 1620, about ______ the USA was covered by forests．
A．a third B．half C．two thirds D．a fourth
（ ）2．A lot of good land has gone with ______．
A．sand B．water C．wind D．forests
（ ）3．The Great Green Wall in China is ______ long．
A．7,000 kilometers B．1,700 kilometers C．7,000 meters D．400 kilometers
( ) 4．Trees must be grown in ______.
A．China B．the USA C．some countries D．every part of the world
( ) 5．______ will make the world better．
A．The Great Wall B．Tall buildings
C．Great Green Walls D．Flowers and grass