The phrase this source in the passage refers to sun wind

Câu hỏi :

Read the following passage, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions. The quest for sustainable sources of energy study the energy has led humans to study the energy potential of the sun and the wind, as well as the immense power created by dammed rivers. The oceans, too, represent an impressive source of potential energy. For example, it has been estimated that the oceans could provide nearly 3,000 times the energy generated by hydroelectric dams such as the Hoover Dam. Yet, this source remains quite difficult to exploit. But this challenge has not prevented scientists from trying. Within the last few decades, several technologies that can transform the ocean’s immense forces into usable electricity have been invented and introduced. Some focus on capturing the power of the changing tides, while others rely on thermal energy created by oceans in certain tropical regions. However, the most common and easiest-to-develop technologies are those designed to harness the power inherent in the ocean’s waves. There are several methods by which ocean-wave energy can be collected. All of them work because the movement of the water that the waves induce creates storable energy by directly or indirectly driving a power generator. In one such technology, the changing water levels in the ocean that are produced by waves lift a long floating tube comprised of many sections connected by hinges. As the sections move up and down with the water, they pump a special fluid through the tube that can be used to drive a generator. Another technique works on a similar principle, only the floating object rocks back and forth with the motion of the water instead of up and down. A third method of collecting wave energy relies on the rising water from the waves to compress air in a partially submerged chamber. As the waves rush into the chamber, they push the air out through a narrow tunnel. Located inside this tunnel is a turbine connected to a power generator. The movement of the air turns the turbine, which feeds energy into the generator. The drawback to each of these concepts is that the they make it necessary to have many pieces of machinery linked together. This presents a problem because the larger the device, the more vulnerable it is to damage from hazardous ocean environments, and the more likely it is to interfere with otherwise unspoiled coastal scenery. Also, these methods demand the construction of site-specific machines that take into consideration average local wave heights and sea conditions. Such a requirement can be quite cost-prohibitive, because engineers must create unique power generation mechanism for each site. In other words, the ability to get power from waves differ from region to region. Japan, Norway, and the UK have attempted to generate energy by capturing the power of ocean waves. In northern Scotland, the first power plan to use wave power, OSPREY ( Ocean Swell Powered Renewable Energy ), began operating in 1995. It followed the principle of the third method described above : waves entering a partially submerged chamber pushed air into turbines to generate electricity. The electricity was then transmitted to power collectors in the shore via underwater cables. Unfortunately, the OSPREY plant was destroyed in a large storm, highlighting an unavoidable difficulty associated with this kind of power generation. The potential benefits of wave-based energy are hard to ignore. Once the proper machinery is produced and installed, the energy is free. Maintenance cost are small, and the equipment does not pose any threats of environmental pollution. And best of all, the amounts of energy produced are enormous. However, these theoretical advantages have yet to be fully realized. In many cases, a lack of government funding has inhibited the technologies from advancing. For example, despite the relative abundance of proposed wave-power devices, many have not been adequately tested, and most have been evaluated only in artificial pools where they are not subjected to the harsh marine conditions that exist in actual oceans. Protecting the equipment from the sea’s destructive forces, as well as the fundamental task of determining feasible locations for collecting energy source are substantial and will require more time to overcome.