user: Question: what round saw Wakefield Trinity Wildcats U21 as the opponenet at Belle Vue?
Columns: ['Round', 'Opponent', 'Date', 'Venue', 'Competition']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['N', 'Whitehaven', '04/02/07', 'Recreation Ground', 'Friendly']
SQL:

assistant: SELECT "Round" FROM "Table" WHERE "Opponent" = 'Wakefield Trinity Wildcats U21' AND "Venue" = 'Belle Vue';