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　　Take，for example，the second 1981 flight of the Space Shuttle Challenger．During that mission，a powerful，experimental radar was pointed at a lifeless stretch of desert in Egypt called the Selima Sand Sheet(pan of the Sahara Desert)．To everyone's surprise，the radar penetrated through the sand to the harder rock beneath．On the surface．there is a 1ittle indication that Africa's Sahara Desert was never anything but a desert．When the archeologists studied the radar images，they saw what seemed to be impossible：there was sand．buried landscape that was shaped by flowing water；traces of ancient riverbeds appeared to be over nine miles wide，far wider than most secti6ns of the present．day Nile River．Today,the area is one of the hottest．Driest desert in the world．

　　Archeologists dug pits along the old river banks and found clues to the past：stream．Rounded pebbles(鹅卵石)，Stone-Age axes，broken ostrich(鸵鸟)eggshells，and the shells of land snails．The archeologists were quite pleased with these findings．For years，they'd been finding stone axes scattered through the desert，and couldn't understand why．Now we know that early humans were living on the banks of old rivers，and left their beautiful tools behind．Some are so sharp that you could shave with them．

　　More recently,Lmadsat 4，a special Earth-mapping satellite，aided in the discovery of ancient Mayan ruins in Mexico.Landsat can．with the help of false-color imagery,"see through"much of the area．Armed with these maps，a five-person expedition took to the air in a helicopter．

　　By the end of the second day,the team found a stretch of walled fields that expedition inembers said look like"old New England fences"．They just go on，non'stop，for 40 miles.Later

　　in the week，an ancient village was pinpointed，as was the"lost"city of Oxpemul，once found in the early 1930's but quickly reclaimed by the jungle．The findings made them able to map the extent of the Mayan civilization in about five days．Working on foot，it would have taken at least 100 years．

　　4l With the help of the space-shuttle-carried radar，archeologists found

　　A  a new stretch of the Sahara desert．

　　B traces of ancient riverbeds under the Sahara Desert．

　　C  some traditional archeological tools in the Sahara Desert．

　　D  a mountain beneath the Sahara Desert．

　　42 Which of the following is true of the sand-buried landscape?

　　A  It was all old avenue．

　　B  It was an underground river．

　　C  It was shaped by flowing water．

　　D  It was shaped by the old Nile River．

　　43 The stream-rounded pebbles and Stone-Age axes which were found along the ancient river banks show that

　　A an early human civilization once existed along the old river banks.

　　B ancient people didn't know how to make weapons．

　　C most species ofimimals in Sahara have disappeared．

　　D early humans were good at fighting with sharp weapons．

　　44 "They"in the second line of the last paragraph refers to

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