user: Question: What is the series number that had 5.31 million viewers?
Columns: ['Series #', 'Season #', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S viewers (million)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: [118.0, 1.0, '"My Mirror Image" (Part 2)', 'John Inwood', 'Tim Hobert', 'November30,2006', 601.0, '8.45']
SQL:

assistant: SELECT "Series #" FROM "Table" WHERE "U.S viewers (million)" = '5.31';