user: Question: How many different original U.S. air dates appear where the U.S. viewers were 2.5 million?
Columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'U.S. viewers (millions)', 'Original U.S. air date', 'Prod. code']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text', 'real']
Sample row: [50.0, 1.0, '"Backstage Rush"', 'Stewart Schill', 'Scott Fellows', '2.3', 'May12,2012', 301.0]
SQL:

assistant: SELECT COUNT("Original U.S. air date") FROM "Table" WHERE "U.S. viewers (millions)" = '2.5';