What kind of cars will be driven in the year 2050? Rather different ones from those that we know today. The next thirty years will bring greater change in car models thomorrow's cars will not look like those of today. The most important change lothe design of cars will be environmental. That is, tomorrow's cars will be designed not to cause air pollution. They will be electrically powered; in other words, they will run on electricity entirely and therefore, be environmentally clean. Besides the problem of pollution, there is also the problem of heavy traffic and traffic accidents today, The last two may also be avoided if computers drive the cars instead of drivers. As a passenger, all you will have to do will be to get in the car and say where you want to go. The computer will do the rest and take you there. This, however, will require the construction of special intelligent roads, as tomorrow's models won't be able to move on ordinary roads. These roads will contain special strips that can provide electrical power to the vehicles as they drive along them. The special equipment in cars will pick up the necessary fuel during long journeys from a power source which will exist in the road. As computers - not drivers - will provide safe driving, there won't be any accidents at all. Which word from the passage best fits the following definition? "to prevent something had from happening"