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阅读下面短文，然后根据短文内容选择最佳答案。
Water can be a liquid, a solid or a gas．When it is a solid, it may be as hard as rock．When it is a liquid, you can pour it out of a bottle．When it is a gas, you cannot see or pr feel it．
Water can change from one form to another easily．When we freeze water, it turns into ice．When we boil water, it turns into steam．When ice melts, it turns into water．When steam cools, it turns into water．The three forms of water are ice, steam and water．
Where does water come from? Water rises to the sky by the heat．Water vapour is formed．This rises and forms clouds in the air．When the clouds come near a mountain, they are forced to rise．As the clouds rise, they are cooled．The higher they rise, the more they are cooled．This causes more and more water vapour to condense(凝结)．In this way, the tiny drops of water in the clouds get bigger and bigger until they are heavy enough to fall to the earth as rain．
Some rain water which falls on the earth again rises to the sky by the sun's heat．Some of it sinks into the ground．It may be used up by thirsty plants．It may reach a well．Most of the water goes back to the rivers, seas and oceans．This process then starts all over again．This process is called the water cycle．
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