We have planted a lot of trees to _____ wind and sand in the desert．

A．hold up                                 B．hold down

C．hold back                              D．hold out

 We have planted a lot of trees to _____ wind and sand in the desert．

       A．hold up                                               B．hold down          

       C．hold back                                            D．hold out

閱讀下列短文，從每題所給的四個(gè)選項(xiàng)中選出最佳選項(xiàng)。

　　In 1620，about half the USA was covered by forests，and in 1850，only about a third．Today a lot of good land has gone with them，leaving only wilder－ness or areas with poor soil．What's worse，too many trees ate still being cut down in the USA．

　　Not copying the USA's example，China has built a new“Great Wall” across the northern part of the country．This time，it is a“Great Green Wall” of trees，millions of trees．It will stop the wind from blowing the earth and the sand away．It will stop the sand from moving southward．It has already saved a lot of land．But more“Great Green Walls” need to be built－not only in China but also all over the world．

　　Wang Feng is a worker at Yulin in Shaanxi．With many other people he works on the Great Green Wall．We visited him at his work place among the young trees and asked him bout his work．“Thousands of trees must be planted every year，” he said．“The more，the better．This year alone，we've already planted ten thousand trees．Just look at all these small trees over there on that hill．They were planted five years ago．But now there is a young forest！In a few years'time，we hope to afforest（綠化）those mountains，too！”He pointed to the high bare mountains far away．

　　“Was it difficult to work on the Great Green Wall？”We asked him thought-fully．

　　“Yes．It's hard work，but it's very important．The only problem is that you can't feed on trees！So we grow our own food，too．We can do it easily now，thanks to the Great Green Wall．”

1．Forests are becoming more and more scarce（稀少）in the USA because

[　　]

A．land is getting poorer and poorer

B．modern agriculture and industry develop too quickly

C．no ways have been found to stop pollution

D．people there have destroyed them during these 350 years

2．China has built a“Great Green Wall” ________．

[　　]

A．in order to keep wind and sand off land and environmental protection in general

[　　]

B．so as to save the desert

C．so that the forests cannot go any longer

D．only to stop the wind from blowing the soil away

3．This text mainly tells us ________．

[　　]

A．a(chǎn)bout the information of forests in the USA and the PRC

B．that more and more“Great Green Walls”are needed in the world

C．how the Great Green Wall came about

D．what has happened to the forests of the USA and China