50、Mi____of elements。There is nothing obvious about the appearance of an element which makes it different from other substances. Many of the most important elements arc not fo und on the earth in a pure state, but have to be prepared in the laboratory.

51、Energy Nearly all energy comes from the sun either in a roundabout way or straigh t from it, in the f____(51)of heat rays and light rays. The light from the moon, too, comes from the sun. The moon can be said to be like a large mirror w____(52)throws back the sun's light to the earth. Electrical energy comes from the sun in a roundabout way; e.g. It can come from the power of waterf____(53)down a mountainside. The water fell there as rain, and we know that rain is made b____(54)the sun's heat evaporating the water on the earth's surface. This water vapor rises, condenses on cooling, and falls as rain. The light and heat energy from coal also c____(55)from the sun in a roundabout way. Coal was made by the rocks pressing on trees and plants which died millions of years ago.T____(56)trees and plants grew with the aid of sunlight, from which they made carbohydrates, in this way changing the sun's energy I____(57)chemical energy. When we burn coal, some of this energy is set free. Energy which we use to d____(58)car engines comes from petrol, which also was made with the aid of the sun in a roundabout way. Plants and animals which ate the plants, d____(59) millions of years ago, and the parts of them that were left were pressed under the rocks in the earth. These parts l____(60)from dead animals and plants made petroleum, from which petrol and oil are now obtained. Thus we can say that the sun is the place where nearly all energy comes fr om, and that without the sun's heat and light, there could be no life on earth.

52、（同51题）

53、（同51题）

54、（同51题）

55、（同51题）

56、（同51题）

57、（同51题）

58、（同51题）

59、（同51题）

60、（同51题）

答案：

1　 D
2　 B
3　 A
4　 B
5　 B
6　 D
7　 B
8　 D
9　 C
10　 C

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