Read the text on renewable energy sources and answer the following questions. There is growing interest and investment in the use of low-carbon technologies to generate electricity. However, all energy production will have environmental side-effects. Our research set out to establish whether renewable energy forms are really better than fossil fuels and nuclear power. In a research paper, we analyzed the environmental impact of electricity generation from renewable sources, nuclear fission power plants and fossil fuels. The environmental impact of any energy source needs to be measured across a range of criteria. We accounted for the environmental effects associated with the production, operation and dismantling of facilities, as well as the production, transport and combustion of fuels. Our study emphatically confirms that fossil fuels – mainly coal – place a heavy burden on the environment and that most renewable power projects have lower pollution-related impacts on ecosystems and human health. Nonetheless, no energy source is without adverse environmental side effects. Photovoltaics (PV) comes out very well in our analysis. Today, the production of PV cells, commonly called solar panels, uses much less energy than previously. The carbon emissions per unit of PV electricity is one-tenth or less of even the most efficient natural gas power plants. However, solar panels require much more space to generate the same amount of power as fossil fuel or nuclear power generators. Also, this need for large tracts of land and can have a negative impact on local species. Does this mean covering huge areas with solar panels is a problem? Not necessarily. The amount of land needed to generate a kilowatt-hour from PV is comparable to that of coal power, when the land associated with mining coal is accounted for. What’s more, about half of the PV installations in our future scenario in 2050 could be placed on rooftops. Environmental effects from hydropower vary widely, we found. Some dams cause significant climate impacts through the emissions of methane from the decomposition of biomass in reservoirs. Other dams cause equally serious ecological problems through habitat destruction. They can also block the migration of aquatic species and reduce sediment flow and nutrient transport, which affects floodplains and deltas. On the other hand, reservoirs form new habitats for birds and other species. Hydropower offers a good example of the importance of site selection and project design. Some projects may be economically viable but ultimately should not be implemented due to the environmental degradation they can cause. For other projects, the impacts can be limited by mitigation strategies such as environmental stream flow and fish ladders, which provide a detour for migrating fish around a hydropower dam. Similar lessons hold for wind power, the first of which is the need to minimise habitat destruction during construction. Then, once facilities are built and up and running, their operations should be adjusted in order to reduce collisions with flying creatures, such as raptors and bats. Also, wind power resources vary widely across locations, which stresses the importance of choosing locations where wind resources are more abundant. Biomass energy, or burning plant material for power, is important in plans to limit global warming. ". When combined with CO₂ capture and storage, it can remove carbon from the air and store it underground, which helps the environment. Using fast growing leafy plants, such as willow and miscanthus, for biomass power can reduce greenhouse gas emissions because they absorb CO2 effectively as they grow. However, growing these plants requires vast areas of land, which can have significant ecological implications, such as the loss of habitat and species. In fact, the ecological damage from biomass energy can be comparable to that from coal and gas. Biomass power is better for ecosystems only when combined with carbon capture and storage. Climate mitigation strategies can provide a rare opportunity to reduce not only carbon emissions but also a wide range of environmental problems. However, deployment of low-carbon technologies should avoid sensitive habitats in order to fully realize their environmental benefits without triggering unintended consequences. Power plant siting, project design and technology choice are critical issues that investors and governments should consider very carefully. While most people recognize that solar and wind are low-carbon energy sources, bioenergy and carbon capture and storage also have an indispensable role in basically all scenarios where countries rapidly reduce carbon emissions. Our results indicate that we need to search for ways to use these technologies while minimizing the harm to ecosystems. What does the underlined word in the text refer to?Write ONE WORD from the text to complete the sentences.23. ‘they’ in paragraph 4 refers to Answer Question 11[input].