Read the passage and choose the correct answer A, B, C or D for each of the following questions. Write your answers on the answer sheet.

When your mobile phone runs out of battery power, all you have to do to get it working again is to charge it up. Imagine if you could do the same thing with your car. Researchers at car companies are developing cars that plug into electric sockets. These vehicles might become the cars of the future. The cars are called plug-in hybrids and they get most of their power from electricity, just like other electrical devices like mobile phones and laptops. This technology would significantly reduce the amount of petrol that people use. For countries that depend on other oil producing countries for their petrol supply this could be a great advantage. Presently, most cars use petrol. Unfortunately, burning fossil fuels produces carbon dioxide gas, and when a large amount of this gas and other greenhouse gases are trapped in the Earth's atmosphere, they cause global warming. Hybrid vehicles have already been developed and are being driven. These cars get their power from both petrol and electricity; they are able to travel longer distances with less petrol. Plug-in hybrids will be even more advanced as they are designed to be powered mainly by electricity and use only a small amount of petrol. It might be some time before the cars will become available to the public but if they do, it will be a major breakthrough for transport.

What is the main idea of the passage?