We live in an era when environmental issues are of great concern. The burning of fossil fuels extracted from the earth has resulted in huge amounts of carbon being released into the atmosphere. At the same time, forests are being 12 by fires. In addition, our waterways are constantly being polluted by chemical 13 from industry and household detergents. The future safety of our water supply is uncertain and many countries may soon be faced with a (an) 14 of clean water.