B

　　(2016·河北省唐山市统一考试)Hybrid cars (混合动力车) are cars that run on petrol and electricity. They have a small standard petrol engine and a battery

　　electric motor to provide electric power.

　　There are some differences between different models of hybrid cars, but the general principle is that the car runs on petrol, and the electric motor kicks in when additional power is required, for example, when going uphill or accelerating. In some hybrid cars the petrol engine turns itself off when not needed, for example when the car has stopped at traffic lights, keeping only the electric engine running.

　　Conventional cars have large engines to deal with driving uphill and accelerating. Hybrid cars have much smaller petrol engines, improved by electric motors when needed, so they use less petrol. Another way that fuel consumption is cut is by a system of “regenerative (再生的) braking”. The electric motor is used to slow down the car, rather than conventional brakes. The energy produced by slowing the car is changed into electrical power, which is automatically stored in the battery. In effect, the battery recharges when you brake. In conventional cars the energy produced when braking is wasted.

　　In 1928, Ferdinand Porsche built an experimental hybrid car. The first mass­produced hybrid car, the Toyota Prius, came out in Japan at the end of 1997. However, hybrid cars became available in the USA only in 1999. As they use less fuel, hybrid cars are cheaper to run. There are also many voices in place to encourage people to buy them. In some countries, hybrid car owners pay a lower rate of tax, and don't have to pay on certain roads. In some cities, hybrid cars are allowed to park for free.

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