user: Question: What is the U.S. air date when the U.S. viewers are 7.5 million?
Columns: ['No.', '#', 'Title', 'Directed by', 'Written by', 'U.S. air date', 'Production code', 'U.S. viewers (million)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: [22.0, 1.0, '"Vortex"', 'Greg Beeman', 'Philip Levens', 'September24,2002', 175051.0, '8.7']
SQL:

assistant: SELECT "U.S. air date" FROM "Table" WHERE "U.S. viewers (million)" = '7.5';