A、It is extremely dangerous to fly in the dark. B、Noise regulations restrict the hours of airport operation C、Some of its runway

The economic costs of noise to society are several. 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.
    If all aircraft were made quieter by existing methods, there would be a number of economic benefits. 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.
    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, properly 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.
19. Why is there no jet traffic at Washington’s National Airport from 11 pm to 7 am?
20. What result might noise reduction cause according to the speaker?
21. What is the passage mainly about?