People are continually searching for diverse sources of energy. At present, the majority of global energy needs are met by burning fossil fuels (1)---- coal, oil, and natural gas. (2)----, a major drawback of fossil fuels is that burning them for energy releases carbon dioxide and other greenhouse gases. For instance, both burning these fuels and extracting them from the earth release substantial emissions. Specifically, extracting the fuels and refining them (3)---- their usable forms itself requires energy, and therefore produces additional carbon dioxide. A further problem is that fossil fuels provide only a one-time burst of energy. Since the supply is finite and will eventually run out, future generations will have to wait millions of years before any more (4)----. This is why people will increasingly need to (5)---- a wider variety of energy sources going forward.