Nghe TOEFL

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Listen to part of a lecture in an environmental science class. The professor is discussing the concept of urban heat islands. "Now, urban areas tend to be warmer than the surrounding rural areas. This phenomenon is known as the urban heat island effect. Why does this happen? Well, one major factor is the replacement of natural surfaces with materials like concrete and asphalt. These materials absorb and retain heat from the sun during the day and release it slowly at night, so temperatures don't drop as much as in rural areas. Another factor is the lack of vegetation. Trees and plants provide shade and release moisture through transpiration, which cools the air. When we remove them, we lose that cooling effect. Also, human activities, such as driving cars and running air conditioners, generate waste heat. This adds to the overall warmth. Interestingly, the intensity of the heat island effect can vary within a city. Dense downtown areas with tall buildings and little green space often experience higher temperatures than residential neighborhoods with trees and parks. So, urban planners are now looking at strategies like planting more trees and using reflective materials on rooftops to mitigate this effect." Now, the professor discusses several factors that contribute to the urban heat island effect. Indicate which of the following is NOT mentioned as a contributing factor.