A、There is a greater demand of clean water. B、More water sources are polluted or disappearing. C、Developed countries consume too

If you live in a city in North America or Europe, you have probably never thought much about water. Whenever you need some, you turn on the tap and there it is. Millions of people in other parts of the world are not so lucky. (29)They have trouble getting enough clean water tot their basic needs. This situation may. soon become common all around the world, scientists believe. In fact, they say that the lack of clean water may be one of the biggest issues in the twenty-first century.
     The reasons for this are clear. (30)On the one hand, people are using more water than ever before. Over the last fifty years, the population of the world has more than doubled. So has the demand for water, for home use, for farming, and for industry. (30)On the other hand, supplies of clean water are disappearing. Many sources of surface water such as rivers, lakes, and streams—are too polluted and unhealthy for use as drinking water. This has forced more and more people to drill wells so they can get water from underground.
    There are enormous amounts of water deep underground in lakes called aquifers. Until recently, scientists believed this groundwater was safe from pollution. Then, in 1980s, people in the United States began to find chemicals in their well water, and scientists took a closer look at what was happening. Weldon Spring, Missouri, for example, was the site of a bomb factory during World War Ⅱ. The factory was destroyed after the war, but poisonous chemicals remained on the ground. Very slowly, (31) theses chemicals dripped down through the ground and into the aquifer. Once they did, however, the water from that aquifer was no longer drinkable.