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At the present time, gas and oil are expensive. Developing countries cannot export__(1)__ agricultural products and other raw materials to __(2)__ the fuel that they need to produce energy. At the same time, petroleum supplies are limited, and in a __(3)__ decades they will run out. However, the supply of sunlight is limitless, and most of the poor countries of the world are in the tropics __(4)__ there is plenty of sunlight. The photovoltaic cell__(5)__ a very important advantage. It is a clean source of energy. The fossil fuels that we use today__(6)__ the main source of the pollution in our atmosphere. It took only a decade __(7)___ scientists to know that solar energy from photovoltaic cell was not just a dream. They have already __(8)___ that it can become an important source of energy. By the end of the century, it will be cheaper to produce electricity with solar cells__(9)__ from petroleum. The photovoltaic cell can be the solution to one of the (10) serious problems in the world today và giải thích