在三相三线制电路中，iA+iB+iC之和（）A、等于0； B、大于0； C、小于0； D

     We've used the wind as an energy source for a long time.The Babylonians and Chinese were using wind power to pump water for irrigating crops 4000 years ago, and sailing boats were around long before

that.Wind power was used in the Middle Ages, in Europe, to grind (磨碎) corn, which is where the term "windmill" comes from.

     We can use the energy in the wind by building a tall tower, with a large propellor on the top.The wind

blows the propellor round, which turns a generator to produce electricity.

     We tend  to build many of these  towers together, to make a "wind farm" and produce more electricity.The more towers, the more wind, and the larger the propellors, the more electricity we can make.It's only worth building wind farms in places that have strong, steady winds, although boats and caravans (大篷车) increasingly have small wind generators to help keep their batteries charged.

     The best places for wind farms are in coastal areas, at the tops of rounded hills, open plains and gaps in mountains-places where the wind is strong and reliable.Some are offshore.To be worthwhile, you need

an average wind speed of around 25 km/h.Most wind farms in the UK are in Cornwall or Wales.Isolated places such as farms may have their own wind generators.In California, several "wind farms" supply

electricity to homes around Los Angeles.

      The propellors are large, to obtain energy from the largest possible volume of air.The blades can be

angled to cope with varying wind speeds.Some designs use vertical turbines (垂直涡轮机), which don't

need to be turned to face the wind.The towers are tall, to get the propellors as high as possible, up to

where the wind is stronger.This means that the land beneath can still be used for farming.