Passage Two
[31]The economic costs of noise to society are several. [29]Airports are currently operating at less capacity because of noise regulations which restrict their hours of operation. For instance, at Washington's National Airport jet traffic is not allowed from 11 pm to 7 am. Other airports restrict the use of certain runways. One estimate is that noise restrictions reduce possible airport use by 20 percent. The profitable cargo trade is especially affected by night restrictions.
In the case of airports, jet engines may be modified to reduce their noise level. One estimate is that $5.7 billion would be required to equip all existing jet engines with noise control devices. However, even taking this step will not reduce noise levels at all points to acceptable values. Some combination of methods is probably necessary.
[30]/[31]If all aircraft were made quieter by existing methods, there would be a number of economic benefits. [30]An increase in airport capacity would occur.Property values near airports might rise. Transportation costs to and from airports could be reduced since the airports now could be located closer to population centers.
[31]Much research still needs to be done on the economic aspects of noise reduction and noise effects. Although some of the effects of noise pollution are known, more must be discovered about its effects on health, productivity, property values and the quality of life. Furthermore, the cost of noise pollution to the economy as a whole needs to be investigated. The public must be alerted to the dangers and economic costs of noise pollution so that people may make intelligent choices and exert appropriate pressures.

Questions 29 to 31 are based on the passage you just heard.
29. Why is there no jet traffic at Washington's National Airport from 11 pm to 7 am?
正確答案：B
解析：細節題。短文開頭提到，由于噪音控制條例限制飛機運作時間，現今機場在以較低的容納量運作，比如，華盛頓國家機場就規定從晚上11點到次日凌晨7點禁止飛機起降。由此可知，B為正確答案。
30. What result might noise reduction cause according to the speaker?
正確答案：A
解析：細節題。短文提到，如果現存方法可以降低飛機噪音的話，將會帶來許多經濟利益；機場容納量將會提升……。由此可知，A為正確答案。B、D與原文相悖；C是noise reduction的原因，而不是結果。題目中多個選項在錄音中都有提及時，要注意適當地做記錄。
31. What is the passage mainly about?
正確答案：D
解析：主旨題。本文并無主題句，需要通過上下文綜合理解。短文首先提出了噪音所帶來的economic costs這個問題，然后又討論了noise reduction所帶來的economic benefits。通過對文章的整體把握，可以推斷出D為正確答案。A、B、C在文中都有提及，均為細節內容而非文章主題。