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?
CREATE TABLE "2013" (
  "date" text,
  "place" text,
  "race_winners" text,
  "gp_winner" text,
  "source" text
);
SELECT "source" FROM "2013" WHERE "race_winners"='ben adriaenssen / ben van den bogaart' AND "gp_winner"='ben adriaenssen / ben van den bogaart' AND "date"='1 april';