Water is important for pupils to study, not only because it is so abundant, covering three times as much of the earth’s surface as land does. It is the most common substance in living plant and animal tissues; without it, most other chemicals needed for life could not get into or out of cells. Its floatability makes it possible to move people and cargoes by ship--without which early exploration and commerce would have been impossible. Hydroelectric(水电) stations are and will continue to be a major source of electrical energy. Water is also important for sanitation (卫生) and recreation.Early experiences with liquids should include those dealing with floatability, change of state, and surface films. Children should investigate under what conditions objects sink or float, both in water and in other liquids. They should watch ice melt, and water freeze, and evaporate. They should also observe other substances that change state easily, such as candle wax and butter. And they should observe the wide range of chemicals that dissolve in water.In their investigations, pupils should become aware of the ease with which certain liquids evaporate and of the danger of strong-smelling substances such as those from cleaning fluids and gasoline. They should experience the pleasure of playing in and around water, but they should also be made aware of the hazards(危险) and be taught to play safely.Pupils must learn of society’s increasing demands upon fresh water and develop a healthy concern for how it is used. The amount of water on earth does not change; it is only recycled. What is used for pupils today is part of the same water used even by Columbus and Cleopatra!Finally, pupils should come to appreciate the different kinds of methods by which fresh water is transported from the sea to land areas. They should know the potential(潜力) of the sea--for all practical purposes, the last frontier on earth--for additional fresh water, for food, and for chemicals. They also should understand the importance of the sea for transportation and recreation, as well as for its effect on weather. It can be inferred from the passage that（）

A. clear water on earth is unlimited.
B. fossil fuels and nuclear energy are more important than falling water from dammed-up rivers
C. part of the water used by a person or a community may eventually be used by others
D. most of the earth’s hydroelectric potential has been exploited