[Read the passage below and mention the most appropriate answer to the question.] Providing enough energy to meet an ever- increasing demand is one of the gravest problems the world now facing. Energy is the key to an industrialized economy which calls for a doubling of electrical output every ten to twelve years. Meanwhile, the day's cheap, abundant and environmentally acceptable power may be coming to an end. Coal is plentiful but polluting, natural gas is scare, and oil is not found everywhere, nuclear power now appears costly and risky. In many countries of the world, keen interest is being shown in new energy sources. Among the familiar but largely undeveloped sources; solar energy, thermal energy and energy from ocean deserve special consideration. Scientists are working to find ways to tap the energy from ocean and sun. If they are fully successful, it will mean that we have an abundant and uninterrupted supply of cheap energy. This energy will be clean and will not contribute to environmental pollution.