Did you know that than South Korea?  America is (large a country such that) it is not difficult to understand ① why is air travel  so popular. In fact more than half the world's airline passengers are Americans. ② Over 420 million people a year fly from one place to another within the United States alone, O'Hare International Airport in Chicago is ③the world's busiest airport. At 0'Hare Airport, ④ an average of fifty planes land and take off every hour. That is nearly ⑤ one plane a minute!