Water is very important to living things．Without water there can be no life on the earth．All animals and plants need water．Man also needs water．We need water to drink．to cool our food and to clean ourselves．Water is needed in offices，factories and schools．
Water is needed everywhere．There is water in seas；rivers and lakes ,Water is found almost everywhere．Even in the driest part of the world．there is some water in the air．You cannot see or feel it when it is a part of the air．The water in the seas，rivers and lakes is a liquid。The water in the air is a gas，and we call it water vapor．
Clouds are made of water．They may be made of very small drops of water．They may also be made of snow crystals(结晶体)．Snow crystals are very，very small crystals of ice．Ice is frozen water．It is a solid．There can be snow and ice everywhere in winter．
( )4．Where Can we find water?
A．We can find water when it turns into vapor．
B．Water can be found almost everywhere
C．We can see water in deserts here and there．
D．Water is only in seas and rivers．
( )5．We must protect water resources because ．
A．crops need water B．all living things need water
C．all animals need water D．all plants need water
，求要使滑块沿着滑槽匀速滑动，需要的水平方向的作用力F的大小。