user: Question: In which round did Roy Salvadori won Class D and Alan Hutcheson won Class B?
Columns: ['Round', 'Circuit', 'Date', 'Class A Winner', 'Class B Winner', 'Class C Winner', 'Class D Winner']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Snetterton Motor Racing Circuit , Norfolk', '25 March', "George 'Doc' Shepherd", 'Alan Hutcheson', 'None (no entries?)', 'Gawaine Baillie']
SQL:

assistant: SELECT "Round" FROM "Table" WHERE "Class D Winner" = 'Roy Salvadori' AND "Class B Winner" = 'Alan Hutcheson';