Passage Three
　　Questions 21 to 25 are based on the following passage.
　　Humanity uses a little less than half the water available worldwide. Yet occurrences of shortages and droughts (干旱) are causing famine and distress in some areas, and industrial and agricultural by-products are polluting water supplies. Since the world's population is expected to double in the next 50 years, many experts think we are on the edge of a widespread water crisis.
　　But that doesn't have to be the outcome. Water shortages do not have to trouble the world-if we start valuing water more than we have in the past. Just as we began to appreciate petroleum more after the 1970s oil crises, today we must start looking at water from a fresh economic perspective. We can no longer afford to consider water a virtually free resource of which we can use as much as we like in any way we want.
　　Instead, for all uses except the domestic demand of the poor, governments should price water to reflect its actual value. This means charging a fee for the water itself as well as for the supply costs.
　　Governments should also protect this resource by providing water in more economically and environmentally sound ways. For example, often the cheapest way to provide irrigation(灌溉)water in the dry tropics is through small-scale projects, such as gathering rainfall in depressions(凹地) and pumping it to nearby cropland.
　　No matter what steps governments take to provide water more efficiently, they must change their institutional and legal approaches to water use. Rather than spread control among hundreds or even thousands of local, regional, and national agencies that watch various aspects of water use, countries should set up central authorities to coordinate water policy.
　　21. What is the real cause of the potentials water crisis.
　　A)Only half of the world's water can be used.
　　B)The world population is increasing faster and faster.
　　C)Half of the world's water resources have been seriously polluted.
　　D)Humanity has not placed efficient value on water resources.
　　22. As indicated in the passage, the water problem _________ .
　　A)is already serious in certain parts of the world.
　　B)Has been exaggerated by some experts in the field
　　C)Poses a challenge to the technology of building reservoirs
　　D)Is underestimated by government organizations at different levels
　　23. According to the author, the water price should _______ .
　　A)be reduced to the minimum
　　B)stimulate domestic demand
　　C)correspond to its real value
　　D)take into account the occurrences of droughts
　　24. The author says that in some hot and dry areas it is advisable to _________ .
　　A)build big lakes to store water
　　B)construct big pumping stations
　　C)build small and cheap irrigation systems
　　D)channel water from nearby rivers to cropland
　　25. In order to raise the efficiency of the water supply, measures should be taken to ______ .
　　A)guarantee full protection of the environment
　　B)centralize the management of water resources
　　C)increase the sense of responsibility of agencies at all levels
　　D)encourage local and regional of water resources