III.阅读理解

　　A

　　Researchers have solved a long­standing mystery：why rocks on a flat lakebed in Death Valley National Park in California sometimes move. Racetrack Playa，the waterless lakebed，is named for the long trails(痕迹)that the rocks，some of which weigh hundreds of pounds，leave in the earth.

　　What was one of the world's natural wonders now appears to be the perfect combination of rain, wind，ice and sun，scientists have found.Dr.Norris and James M．Norris，a cousin and co­author, actually saw the rocks moving in December when they went to check on the project.

　　For their study, they used special global positioning system tools, designed by James Norris，in rocks of various sizes brought from outside the park. They were not allowed to move or change anything of the existing playa stones.

　　No river flows into the playa, though rainwater sometimes fills part of it to a depth of a few inches.It's when this happens, and the nights are below freezing and the days are fine, that the rocks may"sail"，the researchers said. On those occasions, the cold night air leaves a sheet(薄片)of ice only an eighth of an inch thick. Then the warmth of the sun causes the sheet to break up. As more ice melts, some of these lesser sheets have room to move.Driven by light winds, the sheets push up against rocks. The wind drives the water，too，which also helps push the ice to some degree. The movement is slow, no more than about 15 feet a minute.

　　The movements are episodic-the conditions may be just right for a few minutes, and then the sun causes more ice to break and the movement stops.And once the water disappears，it may be years before the rocks move again，even if the playa floods again，because the right temperature and wind conditions may not occur.

　　【语篇解读】　美国加州的赛马场盐湖常年干旱，而上面的石头为什么会移动呢？科学家已经破解了这一自然之谜。

　　1．What do we know about Racetrack Playa?

　　A．It has flat rocks in the earth.

B．It is covered by rocks.