阅读理解。
In 1620, about half the U. S. A. 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 U. S. A. '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.
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
2MgCl2+Ti , 在Ar气体中进行的目的是 