A、Along big rivers. B、Near the sources of rivers. C、Near the sources of lakes. D、Near the sources of fresh water. D短文中提到，历史上，人们一

Three-fourths of the world is covered with water. But only about 3% of it is fresh. All the rest is salt water, which fills the oceans and seas. In history, men have tried to build their homes near the sources of fresh water. Now, fresh water is becoming rare, but more and more is needed because of the increasing number of people. Some industries also use large amounts of fresh water in the production of steel, paper, and so on. Scientists estimate that the need of fresh water will have doubled by the year 2050. If it is the truth, we must find new ways of saving or producing it. Some nations have worked on the problem and are already sharing their information with others. They are trying to keep their rivers from being polluted. Deep wells are also being dug, and rainwater is being collected in huge artificial lakes. In one way or another, they hope to provide enough water to satisfy the needs of their people. One important way to have fresh water is to make it from seawater. In this process, the salty seawater is heated, leaving the salt. When the vapor cools down, it becomes water again. The great problem in desalting is not whether it works, but how much it costs. Building the plants is costly and so are the equipment and fuel for the process. One solution to the problem of the fresh water may be to construct companies with two purposes, that is, making fresh water and generating electricity.
16. Where did humans usually build their houses in the past?
17. How can scientists provide more fresh water for people?
18. What is the main problem in desalting water?