Narrator: Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a surprising solution to urban flooding: green roofs. Now, traditional roofs are impervious, meaning water runs off quickly, overwhelming drainage systems. Green roofs, on the other hand, are covered with vegetation and soil. They act like a sponge, absorbing rainfall and releasing it slowly. But here's the key point: the effectiveness of a green roof depends heavily on the type of soil mix used. A lighter mix with more organic matter retains more water, but it can dry out quickly in summer, stressing the plants. A heavier, mineral-based mix holds water longer, but it's less absorbent overall, so more water runs off during heavy rain. So engineers are experimenting with blends, trying to find the optimal balance. Another factor is the depth of the substrate. Deeper green roofs can store more water and support a wider variety of plants, but they require stronger structural support, which increases building costs. So, in retrofitting older buildings, shallow roofs are often the only option, limiting their water retention capacity. This trade-off between cost and environmental benefit is a central challenge in sustainable urban design.
❓ Pregunta What is the professor's main point about green roofs?
A. A. They are too expensive for most urban buildings.
B. B. Their effectiveness depends on several design factors.
C. C. They are not useful in areas with heavy rainfall.
D. D. They require more maintenance than traditional roofs.
✅ Respuesta correcta
B. Their effectiveness depends on several design factors.
TOEFL Listening Tips
Skim the questions first to predict the content
Watch for signal words in lectures and conversations (however, actually, in fact)
For main idea questions, focus on the beginning; for detail questions, focus on examples and data
For attitude questions, pay attention to the speaker's tone and word choice
Take notes on keywords; don't try to write down every word