user: Question: What is the highest production code of the episodes having a US viewership of exactly 2.3?
Columns: ['No in. series', 'No in. season', 'Title', 'Original air date', 'Production Code', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'real', 'text']
Sample row: [21.0, 1.0, '"A Doomed Christmas"', 'December10,2011', 121.0, 'N/A']
SQL:

assistant: SELECT MAX("Production Code") FROM "Table" WHERE "U.S. viewers (millions)" = '2.3';