题目原文：
Hail—pieces of ice that form and fall from clouds instead of snow or rain—has always been a problem for farmers in some areas of the United States. Hail pellets can fall with great force and destroy crops in the field. Over the last few decades, a method of reducing hail, called “cloud seeding,” has been tried. In cloud seeding, the chemical silver iodide is sprayed on storm clouds from an airplane. This makes the clouds produce harmless rain or snow instead of hail. Several pieces of evidence suggest that cloud seeding has been effective in protecting crops from hail.
Laboratory experiments
Experiments in the laboratory support the idea that cloud seeding is effective. Hail usually forms in water vapor that is close to the freezing point. However, when experimenters added silver iodide to cold water vapor in the laboratory, they often observed light snow forming instead of hail pellets.
Evidence from Asia
There is evidence about the effectiveness of cloud seeding from several countries around the world. In some Asian countries, for example, cloud seeding has been successfully used to control precipitation in urban areas. These positive results suggest that cloud seeding should also be effective in protecting fields and farms in the United States.
Local studies
A few local studies also support the value of cloud seeding. One study conducted in a farming region in the central United States, for example, directly monitored crop damage due to hail. The study found that in an area where cloud seeding was used there was reduced hail damage compared to previous years.
题目音频：
学术写作：
简化大意：
教授：为了减少空气污染，是否应该向航空收环保税。
Andrew：应该，可以让乘客减少飞机出行。
Claire：不应该，让乘客承担贵机票不对。
详细题目回忆：
Doctor Achebe:
This week, we’ll be discussing ways to address air pollution. One idea is introducing environmental taxes on air travel. Such taxes, also referred to as ecological taxes or green taxes, are extra charges that get added to the price of airplane tickets. Money collected from these taxes could be invested in environmentally friendly technologies or in promoting other, cleaner modes of transportation. Do you think there should be an environmental tax on air travel? Why or why not?
Andrew:
We should impose environmental taxes on air travel. These taxes might encourage people to find cheaper, more eco-friendly ways to get places. In my country, many people use planes to fly a short distance because flights are relatively cheap when they could just as easily take a less polluting form of transportation.
Claire:
Additional taxes on air travel will do little to solve the problem of air pollution. Airplane tickets would become more expensive, punishing passengers without incentivizing airlines to become more environmentally friendly. The airlines should pay the price to protect the environment, not their customers.
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