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?
SQL: SELECT source FROM table_name_8 WHERE race_winners = "ben adriaenssen / ben van den bogaart" AND gp_winner = "ben adriaenssen / ben van den bogaart" AND date = "1 april"