The world's top automakers(汽车制造商)are increasingly offering more electric car models. This growth is expected to continue, with more people choosing to hit the road with clean- running electric cars.

Environmentalists have praised the automakers for taking major steps to limit harmful pollutants linked to worldwide climate change. But electric cars are also known for not producing another kind of pollution - noise. They run on batteries instead of fuels, and can operate in silence.

While many people might consider this a good thing, quiet cars can also cause problems. The main danger is that people around electric vehicles face a greater risk of being hit if they cannot hear the cars coming.

Governments in the United States and Europe have recognized this problem. So they have set requirements for automakers to add warning sounds to electric vehicles. The U.S. Department of Transportation finalized its rules a year ago. The rules require electric and hybrid (混合物) cars to be equipped with some warning sounds when moving at a speed of up to 30 kilometers per hour. The rules are aimed at preventing injuries among people walking or riding bicycles and protecting the blind.

So what kind of sound can we expect to hear from the next generation of electric cars? One of the easiest solutions would be for carmakers to reproduce the sound of a traditional car running on fuels. But many industry officials believe this would be a mistake. They say this would not support electric cars' true identity. Frank Welsch, the head of technical development at Volkswagen, said that finding the perfect sound mix is not an easy process. He said, "It cannot be too annoying. It cannot sound like anything we had in the past." An official from Mercedes-Benz said the sound was designed to provide a safe warning without annoying passengers inside the car. He said the goal was to create a car that remains completely quiet inside.

It might seem strange to think that part of our automotive future is currently being developed in the same kinds of recording studios used by musicians. But that is exactly what is happening.

Whenever I moved to a new city, the first thing I did was to hire a bicycle. Why? 【小题1】 You travel much faster than on foot and you notice so much more of your surroundings than when in a bus or a car. It's more appealing to wander down small streets if they seem interesting, as you can quickly get back to the main road. Besides, I soon discovered more benefits of cycling.

A bicycle means that fewer places, especially in a small city like the one where I live now, feel far away. For example, cycling to work takes me half the time that it would to walk, 【小题2】

Of course, there are obvious health benefits of cycling — a bit of extra movement as part of your daily routine can help you keep fit and healthy. Cycling on my way to work also makes me feel more energetic in the mornings.

【小题3】 While cars pollute the environment，bicycles are carbon neutral (碳平衡的). Furthermore, thanks to being petrol-free, bicycles are cheaper to run, buy and fix than cars. Even if you don't own a car，a bicycle can pay for itself when compared to the price of bus tickets.

【小题4】 Always wear a helmet, pay attention to other traffic and don't forget your lights at night. Make sure you know the road rules, especially in a new place. Some cities are more bicycle-friendly than others. 【小题5】 Luckily, where I live now is quite easy to ride a bicycle around, thanks to having lots of cycle lanes and not too many hills!

In Denmark, bicycling is one of the primary forms of transportation. In sunshine, rain and snow, you will see cyclists on their way to work, to the grocery store, or to social events. The bike is a Dane’s best friend.

Danish cycling culture is as old as the bicycle itself. Bikes were first introduced to the country in the 1880s, and during the 1920s and 1930s, the bicycle became a widespread symbol of equality and freedom. People of all social classes began biking side by side — in the cities on their way to work and in the countryside on their days off.

The increased economy development of the late 1950s saw some Danes replacing bikes with motorcycles and automobiles. Just like their colleagues around the world, Danish urban planners believed the future belonged to cars, trucks, and ever-wider highways. In the early 1970s, however, the Mideast oil crisis terminated that development. “Car Free Sundays” were introduced in Copenhagen, and movements demanding Copenhagen to become car-free took place. Over time, concerns about air pollution, climate change, and the need for people to get enough exercise have helped bicycles make a big comeback. Denmark’s heavy taxes on petrol and automobiles are a factor, too.

Today, cycling is an inseparable part of Danish culture. Newcomers who do not know how to cycle are encouraged to learn as soon as they arrive. Danish children usually learn to bike before they begin school at the age of six — and often much earlier. In school, children learn about traffic rules, road safety, and the importance of wearing a helmet as well as good cycling habits. Another choice for families with children is the cargo bike — a sort of oversized tricycle with a large wooden box on the front. It’s estimated that a quarter of all Copenhagen families with two or more children own one of these cargo bikes for transporting kids, groceries, and other necessities. Danish cargo bikes have also won design awards and become a Danish export success.

Commuting by bike is the fastest, easiest and most environmentally friendly way to get around the cities of Denmark. And the numbers speak for themselves: Residents who cycle in Copenhagen request 1.1 million fewer sick days. Cyclists reduce CO2 emissions by 20,000 tonnes a year on average.