user: Question: What is the production code for the episode that had 23.9 million u.s. viewers?
Columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: [98.0, 1.0, '" The One After Ross Says Rachel "', 'Kevin S. Bright', 'Seth Kurland', 'September24,1998', 467651.0, '31.1']
SQL:

assistant: SELECT "Production code" FROM "Table" WHERE "U.S. viewers (millions)" = '23.9';