【正确答案】Key points: ·The lecture states that hydropower facilities are not always convenient for everyone because some people are forced to move when a dam is built in their community. This contrasts with the point in the reading that hydropower reservoirs contribute to the local economy and quality of life. ·The lecture states that hydroelectricity causes long-term environmental hazards and toxic waste in the buildup of silt behind the dams, which will eventually be expensive to clean up. This contrasts with the point in the reading that hydroelectricity is "clean and green" because it does not release greenhouse gases into the atmosphere nor produce waste that requires special containment. ·The lecture states that the huge dams built for hydroelectricity remove water needed for healthy river ecosystems, harming bird and fish populations. This contrasts with the point in the reading that hydroelectricity is "clean and green." ·The lecture states that even though water is a naturally recurring resource, the water supply is subject to natural conditions that make water as a source of fuel unreliable in the long run. This contrasts with the point in the reading that the earth's water cycle naturally replenishes the supply of fuel water.

【答案解析】[解析] Now listen to part of a lecture on this topic in an ecology class. The advantages of hydropower are well known, but there are also some serious disadvantages that should cause us to rethink its overall benefit. First, hydropower facilities are not always convenient for everyone involved. When a dam is built on a river, water floods the land behind the dam. This forces the people who live in the flooded area to move. Forcing people to move is often bitter and controversial, especially if a community has maintained a particular way of life on the same land for several generations. Second, utility companies often emphasize that hydroelectricity is environmentally friendly, "clean and green," but they fail to mention the long term environmental hazards. Dams hold back silt and other debris, which collect behind the dam on the river bottom. This causes an accumulation of toxic heavy metals and other polluting substances, showing us that hydropower does indeed produce toxic waste. The buildup of silt means that future generations will have an expensive and dangerous mess to clean up. Third, the huge dams built for hydroelectricity change a river's natural functions. By diverting water for power, dams remove water needed for healthy river ecosystems. The section of river below a dam often dries up, and birds that migrate there no longer have enough insects to feed on when the water level drops. Also, fish such as salmon depend on a steady flow of cool, fresh water. Native fish populations may decrease or disappear because of the warmer and slower-moving water caused by dams. The environmental changes caused by hydropower may not be obvious right away. Most people will notice air pollution around a coal-burning power plant, but they won't notice a smaller bird population near a hydropower plant. While it's true that hydropower is a renewable energy source, it often carries a heavy environmental cost. Furthermore, even though water is a naturally recurring product, the water supply is subject to natural conditions, such as changing weather patterns and regional drought, so water as a source of fuel may not be reliable in the long run.