下面有3篇短文，每篇短文后有5道题，每道题后面有4个选项。
第一篇
(2002年理工A级阅读理解考题)

Space-Age Archeology It’s a strange partnership, but a very effective one: Satellites and space-shuttle-carried radar are helping archeologists. How By "seeing" through sand or through treetops to locate important archeological sites. The traditional tools for archeologists are shovels and picks. But high technology is the archeologist’s work and time far more productive. Take for example, the second 1981 flight of the Space Shuttle Challenger. During the mission, a powerful, experimental radar was pointed at a lifeless stretch of desert in Egypt called the Selima Sand Sheet(part of the Sahara Desert). To everyone’s surprise, the radar penetrated through the sand to the harder rock beneath. On the suce, there is a little 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 sections 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, Landsat 4, a special earth-mapping satellite, aided in the discovery of ancient Mayan ruins in Mexico. Lansat 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 members 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.

The stream-rounded pebbled and Stone-Age axes which were found along the ancient river banks show that ______.