user: Question: What is channel 33.7's official call sign?
Columns: ['Call sign', 'Analog channel', 'Digital channel', 'Virtual channel', 'Network', 'Station Ownership']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['KTCA', 'none', '34', '2.1', 'PBS -tpt', 'Twin Cities Public Television']
SQL:

assistant: SELECT "Call sign" FROM "Table" WHERE "Virtual channel" = '33.7';