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 sue. 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 potential water crisis?

　　A) The world population is increasing faster and faster.

　　B) Half of the worlds water resources have been seriously polluted.

　　C) Humanity has not placed sufficient value on water resources.

　　D) Only half of the worlds water can be used.

　　22.As indicated in the passage, the water problem ________.

　　A) has been exaggerated by some experts in the field

　　B) is underestimated by government organizations at different levels

　　C) poses a challenge to the technology of building reservoirs

　　D) is already serious in certain parts of the world

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