user: Question: What is the production code for the episode with 5.5 million u.s. viewers?
Columns: ['№', '#', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [57.0, 1.0, '" The Freshman "', 'Joss Whedon', 'Joss Whedon', 'October5,1999', '4ABB01', '5.8']
SQL:

assistant: SELECT "Production code" FROM "Table" WHERE "U.S. viewers (millions)" = '5.5';