Read the last paragraph, and choose which sentence is most suitable as a paragraph leader.……………………… 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.