Humanity uses a little less than half the water available worldwide. Yet occurences of shortages and droughts (干旱)are causing famine and distress in some areas, and industrial and agricultural by-products are polluting water supplies. Since the world's population is expected to double in the next 50 years, many experts think we are on the edge of a widespread water crisis.
But that doesn't have to be the outcome. Water shortages do not have to trouble the world- if we start valuing water more than we have in the past. Just as we began to appreciate petroleum more after the 1970s oil crises, today we must start looking at water from a fresh economic perspective. We can no longer afford to consider water a virtually free resource of which we can use as much as we like in any way we want.
Instead, for all uses except the domestic demand of the poor, governments should price water to re- fleet its actual value. This means charging a fee for the water itself as well as for the supply costs.
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 use. 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.
What is the real cause of the potential water crisis？

A. The world population is increasing faster and faster.
B. Half of the world's water resources have been seriously polluted.
C. Humanity has not placed sufficient value on water resources.
D. Only half of the world's water can be used.