_____ Firstly, security threats (such as 9·11 Event) and economic downturns (e.g. the 2008 global economic crisis) have an impact on the number 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 present 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.