四、閱讀測驗【請在下列各題中選出最適當的答案】Water is our most important natural resource. Yet, although it covers most of the world, only 2.5% of it is salt-freeand suitable for human use. Demand for fresh water has risen sharply in the last 50 years, and it is still rising and causingserious problems. Finding possible and effective solutions may be one of the biggest challenges of the 21st Century.There are several reasons behind this growing crisis. The first is waste. About 70% of our fresh water is used togrow crops. It takes 1,000 tons of water to grow just one ton of wheat. Unfortunately, around 60% of that water is wasted.Better irrigation methods are needed to ensure the smart and efficient use of water.Pollution is another big problem. Many of the world’s great rivers and lakes are badly polluted with garbage andchemicals. Hundreds of millions of people relying on those water bodies are thus affected. Steps are being taken to cleanup some rivers and lakes, but they are expensive and can take many years.Overuse also puts pressure on water supplies. In the U.S., 95% of its fresh water comes from underground sources.Levels are quickly falling, since so much water is used to grow crops and raise livestock. Once used, those supplies aregone forever because they are mostly not refilled by rainwater. The key is to lower demand and reduce use.Today around the world, more than one billion people have no access to clean water, which leads to millions ofdeaths every year, including thousands of children dying every day in Africa. By 2025, as many as 25 African countriesmay face water shortages, and wars could even break out over water rights. The fresh water crisis is not limited to poorcountries. Indeed, rich and poor countries from Asia to Europe to America are troubled by the lack of water, a growingproblem that could soon affect us all.
21. What is the main idea of this article?
(A) We should grow crops that won’t need much fresh water.
(B) The lack of fresh water is a problem that keeps getting bigger.
(C) Scientists have invented new ways to save fresh water.
(D) African countries are the hardest hit by shortage of fresh water.