5. ……………………… Firstly, security threats （such as 9/...

5. ……………………… Firstly, security threats (such as 9/11) and economic downturns (e.g. the 2008 global economic crisis) have an impact on numbers of air passengers, although these have always been followed by a rapid recovery in confidence. In addition, there is a lack of infrastructure to support such growth - more airports and runways are needed, but environmental concerns make it difficult to bring about improvements in infrastructure. Most seriously, there are signs that after a period of economic liberalisation, with the removal of barriers to international trade, we may be moving into a more protectionist era, and this may represent the greatest threat to continued growth in air travel.

A、Several recent events have caused a temporary dip in air travel.

B、There are several issues that might slow down or even reverse this growth.

C、There are a number of negative impacts of air travel, such as air pollution.

D、