阅读下面材料，在空格处填入适当的内容（1个单词）或括号内单词的正确形式。

Using Garbage as a Fuel

    Garbage can be a good fuel to use. The things in garbage do not look ____coal, petroleum, or natural gas, but they are chemically similar to these fossil fuels. As we use up our fossil fuel supplies, we might be able to use garbage as ____energy source. Burning garbage ____( be) not a new idea. Some cities in Europe and the United States ____(burn) garbage for years. The heat that is produced by burning garbage is used to boil water. The steam that is produced is used to make electricity or to heat nearby ____( build). In Paris, France, some power plants burn almost 2 million metric tons of the city's garbage each year. The amount of energy ____(produce) is about the same as would be produced by burning almost a half million barrels of oil!

    But there are problems in using garbage as a fuel. Garbage that burns ____(easy) , such as food and paper, must be separated from metals, glass, and other materials ____do not. Another problem is that burning garbage can pollute the air. Our fossil fuel supplies are ____(limit) . Burning garbage might be one kind of energy source that we can use to help meet our energy needs. This method could also reduce the amount of garbage ____(pile) up on the earth.