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　　Eleven countries are building a great wall of trees across Africa. It is called the Great Green Wall. The aim is to bring life to the dry lands and help to fight the bad environment.

　　The wall is 8, 000 kilometers long and 15 kilometers wide. It covers from Senegal in the west to Djibouti in the east, on the south of the Sahara Desert (撒哈拉沙漠). So far, Senegal has made the best progress by planting about 11,000,000 trees.

　　Many years ago, there were few trees in that area and the winds used to be very strong. It was harmful to the fields. Also, there was little rain, so few plants could grow and many people went hungry. Since the tree planting project started, the trees have provided food for the local people and increased the wetness (濕度) of the air. Little by little, there appeared green fields for people to grow plant.

　　The project which began in 2007 is supposed to cost a total of 8, 000, 000, 000 dollars. Many countries are trying their best to help to continue the planting. So we will be able to see the whole green wall across Africa in the near future!

　　71. Why are the countries building the wall of trees?

　　They want to bring life to the dry lands and help to _______________.

　　72. Where is the great wall of trees?

　　It's ____________________ of the Sahara desert.

　　73. Why couldn't many plants grow in that area many years ago?

　　Because there were few trees, strong winds and ________________.

　　74. When did the project begin?

　　It began _____________.

　　75. What does the passage mainly talk about?

　　It mainly talks about _____________________________in Africa