user: Question: What source has a race winner of Ben Adriaenssen / Ben van den Bogaart, and the gp winner of Ben Adriaenssen / Ben van den Bogaart, for the date of 1 April?
Columns: ['Date', 'Place', 'Race winners', 'GP winner', 'Source']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['1 April', 'Frauenfeld', 'Ben Adriaenssen / Ben van den Bogaart', 'Ben Adriaenssen / Ben van den Bogaart', 'Result']
SQL:

assistant: SELECT "Source" FROM "Table" WHERE "Race winners" = 'Ben Adriaenssen / Ben van den Bogaart' AND "GP winner" = 'Ben Adriaenssen / Ben van den Bogaart' AND "Date" = '1 April';