user: Question: What round was Adelaide United the opposition with an attendance of 18,345?
Columns: ['Date', 'Round', 'Attendance', 'Opposition', 'Stadium']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['26 August 2007', 'Round 1', '14,421', 'Melbourne Victory', 'Westpac Stadium']
SQL:

assistant: SELECT "Round" FROM "Table" WHERE "Opposition" = 'Adelaide United' AND "Attendance" = '18,345';