完形填空

March 12 is Tree Planting Day．Do you know how 　　1　　 a tree．First, the 　　2　　 must be just right－neither too wet nor too 　　3　　．It’s best to plant trees in 　　4　　 because it’s 　　5　　．Dig a hole 　　6　　 for the tree．But the hole should not be 　　7　　 deep．Second, knock a long, strong stick into the earth 　　8　　 to the hole．　　9　　 sure that it is straight．Then put the tree in the hole 　　10　　 it is straight．　　11　　that, you should put the 　　12　　 back in the hole again, and 　　13　　 it down hard with your foot several times．Tie the tree 　　14　　 the top of the stick to keep it straight．As last you should water it well, as 　　15　　 as possible．

　　Strong winds, sand in the air, poor visibility(能见度), we all know the characteristics(特点)of a sandstorm．But what else do you know about them?

　　Do you know for example, that the first sandstorm of 2009 hit north China's Inn Mongolia late February?

　　Sandstorms are today a special treat of life in northern China because of desertification(沙漠化)and the retreat(退化)of northern grasslands．They usually occur(出现在)between February and May．

　　Fortunately, sandstorms this year should be fewer than in past years in north China, according to the National Meteorological Center(中央气象台)．

　　And the chances of sandstorms hitting Beijing are small, because recent rain has stopped drought(旱灾)．Tree planting, and other measures taken by the government, has also helped decrease the chance of sandstorm in the city．

　　Sandstorms can be dangerous．However, there are measures you can take to protect yourself from harm．

　　◆Wear a mask．Cover your nose and mouth with a mask that can keep out sand, or use a wet handkerchief．

　　◆If you are driving and the storm is far away from you, it may be possible to outrun it．If it looks like you will be caught in the storm, stop and wait it out．

　　◆Take cover．If there is no shelter, then lie down．Keep eyes, nose and mouth covered．Cover your head with your arms or a backpack to protect yourself against flying objects．

　　If you are caught in a desert sandstorm, take the following action．

　　◆Mark your direction before lying down．It is easy to get lost in a desert．

　　◆Keep plenty of water at hand．If you get lost, you need water to keep your life until you find your way or help arrives．

　　◆Stay together if traveling in a group．Lock arms if caught in a sandstorm．

　　The most useful measure would actually be to make sandstorms disappear forever．To make this goal come true, people should plant trees, and stop desertification．

　　Today, the straight-line distance between Tian'anmen Square and a desert called Tianmo in Hebei province is only about 80km．If desertification is not stopped, environmental protection experts say, it will probably not be long until Beijingers can catch a camel to work．