Read the following paragraph and underline the topic sentences.
Alternative energy sources are becoming increasingly attractive as the energy crisis becomes more severe. Solar heating systems, which use the sun's radiation as a source of energy, are a promising alternative energy source. Nuclear power plants are already in operation in several parts of the country. Government and private industry are even investing the possibility of capturing the power of ocean waves and tides for conversion into usable energy. Coal is once again becoming an acceptable fuel as the nation searches for solutions to the energy shortage. Even garbage is seen as a potential source of energy. In some communities , garbage is burned to heat buildings and light city streets. In short, all these sources of energy will become available in daily life in the future.