Water

　　The earths supply of water probably remains fairly constant in quantity. The total quantity of water is not known very accurately, but it is about enough to cover the surface of the globe to a depth of about two and three-quarter kilometers. Most of it is in the form of the salt water of the oceans - about 97 per cent. The rest

　　Water vapor in the atmosphere is the channel through which the whole water

　　31　Liquid water only exists

　　C　in the coastal areas of the earth.、D　in a very narrow range of temperatures.

　　A　has greatly increased in recent years.、B　remains almost unchanged.

　　33　Most of the fresh water on Earth

　　B　is stored underground.、C　is found in rivers and lakes.

　　34　The word fraction in the second paragraph means

　　D　a major source of information

　　A　over the mountains than over the rivers.、B　over the rivers than over the mountains、C　over the land than over the oceans.、D　over the oceans than over the land.

　　31 C　　32 A　　33 B　　34 A　　35 D