题目内容：
阅读材料，回答题：Renewable Energy Sources
Today petroleum( 石油) provides around 40% of the world's energy needs, mostly fuellingautomobiles. Coal is still used, mostly in power stations, to cover one-quarter of our energy needs,but it is the least efficient, unhealthiest and most environmentally damaging fossil fuel( 矿物燃料). Natural gas reserves could fill some of the gap from oil, but reserves of that will not last into the 22nd century either. Most experts predict we will exhaust easily accessible reserves within 50 years. Less-polluting renewable energy sources offer A．more practical long-term energy solution. "Renewable" refers to the fact that these resources are not used faster than they can be replaced.
Hydroelectric(水力发电的)power is now the most common form of renewable energy, sup-plying around 20% of world electricity. China's Three Gorges Dam is the largest ever. At five timesthe size of the U. S. 's Hoover Dam, its 26 turB．ines(涡轮机) will generate the equivalent energyof 18 coal-fired power stations. It will satisfy 3% of China's entire electricity demand.
In 2003, the first commercial power station to use tidal ( 潮汐的) currents in the open seaopened in Norway. It is designed like windmill( 风车), but others take the form of turB．ines.
As prices fall, wind power has become the fastest growing type of electricity generation--quadrupling(翻两番) worldwide between 1999 and 2005. Modern wind farms consist of turbinesthat generate electricity. Though it will be more expensive, there is more than enough wind to pro-vide the world's entire energy needs. Wind farms come in onshore and offshore forms. They can of-ten end up at spots of natural beauty, and are often unpopular with residents. And turbines are nottotally harmless--they can interfere with radar, alter climate and kill seabirds. Scotland is build-ing Europe's largest wind farm, which will power 200,000 homes. The U. K. 's goal is to generateone-fifth of power from renewable sources, mainly wind, by 2020. but this may cause problems,because wind is unreliable.
According to the passage, which of the following is the most polluting energy resource? A．Petroleum.
B．Coal.
C．Natural gas.
D．Water.
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