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There is not enough oil in the world now. As time goes by， it becomes less and less， so what are we going to do when it runs out? Perhaps we will go back to using horses， carriages and bicycles. In the Second World War， some people didn't use gas made from petroleum (石油) in their cars. They made gas from wood and plants instead. The car didn't go fast， but they ran， so this was better than nothing. However， in the future， we can't cut down all our trees to make gas; we need our trees for other things， too. Besides different types of gas， we can also use electricity (电) to run our cars， but first we must make the electricity! Some countries have coal and they make electricity with that， but we might not always have coal， either. Other countries have big， strong rivers， and they can use the power of water to turn turbines (涡轮机) and make electricity more easily and cheaply. We are also able to get power from the ocean tides. We put turbines into the mouth of the river. Then the tide comes in， the water turns the turbines and then it goes out， it turns them again. Which of these ways will be used to run our cars in the future? 51. When might people have to go back to using horses and carriages? A. When they are poor. B. When they run out of oil. C. When they need more exercise.D. When there aren't any big trees in the world. | 
