【正确答案】
D
【答案解析】[听力原文]
A cloud consists of tiny drops of water or ice crystals. It floats in the air. Clouds are grouped into classes according to their height above the ground. There are three major groups of clouds: there are low clouds, there are middle clouds, and there are high clouds.
The low clouds float less than 6,000 feet or 1,800 meters above sea level. There are two kinds of low clouds. The two kinds of low clouds consist of stratus and stratocumulus clouds. The stratus clouds cover the sky like a white blanket. They bring rain or snow. The stratocumlus clouds cover the sky with large clouded masses.
The middle clouds usually lie from 6,000 to 20,000 feet above sea level. 6,000 to 20,000 feet is about 1,800 to 6,100 meters above sea level. Middle clouds are grouped into three sub- classes. They are grouped into altostratus, altocumulus, and nimbostratus. An altostratus cloud forms a smooth white sheet across the sky. Nimbostratus forms a smooth grey layer across the sky.
In the third category of clouds are the high clouds. High clouds are found 20,000 feet above the earth, or higher.
There are three kinds of high clouds. They are cirrus clouds, cirrostratus clouds, and cirrocumulus clouds. All high clouds are formed entirely of ice crystals. The middle and low clouds are mainly water droplets.
No two clouds are exactly the same. They are always changing their shape or form. Clouds are interesting to scientists, to artists, and to picnickers.