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　　Passage Three

　　The water we drink and use is running short in the world. We all have to learn how to stop wasting our limited water. One of the steps we should take is to find ways of reusing it. Experiments have already been done in this field. Today in most large cities， fresh water is used only once， then it runs into waste system. But it is possible to pipe the used water to a purifying factory. There it can be filtered and treated with chemicals so that it can be used again， just as it were fresh from a spring. But even if every large city purified and reused its water， we still would not have enough. Then we could turn to the oceans. All we'd have to do to make use of the seawater on earth is to get rid of the salt. This process is called desalinization， and it is already in use in many parts of the world.

　　71. The way to stop wasting our limited water is to ________.

　　A. do experiments with water B. purify the used water and reuse it

　　C. use fresh water once again D. make use of seawater

　　72. The following tells us how to reuse the used water. Which is the right order of the passage？

　　A. to have the used water filtered B. to put chemicals in it

　　C. to pipe it to the users D. to pipe the used water to be purified to a factory

　　73. There wouldn't be enough water for us if we didn't _________.

　　A. turn to the oceans for more water B. reuse used water and make use of seawater

　　C. desalt seawater D. take steps to reuse all water on earth

　　74. The would "it" in the last sentence refers to _______.

　　A. seawater B. purified water

　　C. the process of getting rid of the salt in seawater D. the process of collecting salt from the sea oceans

　　75. The best title for the passage is "_______".

　　A. How to Reuse Water B. Two Solutions to the Problem of Water Shortage

　　C. Stop Wasting Our Limited Water D. How to Make Use of Seawater