user: Question: When there are  3.39 million u.s viewers what is the production code?
Columns: ['No.', '#', 'Title', 'Directed by', 'Written by', 'U.S. air date', 'Production code', 'U.S. viewers (million)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [154.0, 2.0, '"Plastique"', 'Rick Rosenthal', 'Don Whitehead & Holly Henderson', 'September25,2008', '3T7452', '4.18']
SQL:

assistant: SELECT "Production code" FROM "Table" WHERE "U.S. viewers (million)" = '3.39';