CREATE TABLE table_name_8 (source VARCHAR, date VARCHAR, race_winners VARCHAR, gp_winner VARCHAR)
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?
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"