Find the id of routes whose source and destination airports are in the United States.
return routes ;return source airports of #1 ;return destination airports of #1 ;return where are #2 ;return where are #3 ;return #1 where #4 is in  the  United States ;return #1 where #5 is in  the  United States ;return #1 in  both #6 and  #7 ;return ids of #8
['select', 'project', 'project', 'project', 'project', 'comparative', 'comparative', 'intersection', 'project']