user: Question: Which broadcast date has 6.8 million viewers?
Columns: ['Episode', 'Broadcast date', 'Run time', 'Viewers (in millions)', 'Archive']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['" Episode 1 "', '23December1967', '23:45', '6.8', '16mm t/r']
SQL:

assistant: SELECT "Broadcast date" FROM "Table" WHERE "Viewers (in millions)" = '6.8';