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The water and the land are thought part of the earth surface (表面). The air is 1 thought a kind of blanket (毯子) 2 the earth. But it is 3 than that. Maybe you have been in a cave (洞穴) 4 in the earth. Did you think about the air that was in the cave? The land has some air mixed in it. Air is even mixed 5 the water in the sea. These examples show that some air is 6 the earths surface as well as above it. Men once 7 that there were four basic things from which everything else was made. They 8 these things-earth, fire, air and water the four elements (元素). 9 man made more observations, they 10 that fire was not an element. 11 they concluded (得出结论) that land, air and water were not elements, 12 . 13 , there are more than 100 elements from which scientists believe all things are made. 14 land, air and water are not elements, they are three main parts of mans environment (环境). You will 15 more about them as you study the earth. 1. A. usually B. seldom C. never D. sometimes 2. A. above B. around C. across D. among 3. A. better B. more C. less D. worse 4. A wide B. high C. long D. deep 5. A. at B. in C. with D. to 6. A. below B. before C. between D. beside 7. A. found B. thought C. knew D. understood | 
