What was the weight in kg when the jockey was B. York? 
CREATE TABLE "1996_97_season_as_a_three_year_old" (
  "result" text,
  "date" text,
  "race" text,
  "venue" text,
  "group" text,
  "distance" text,
  "weight_kg" text,
  "jockey" text,
  "winner_2nd" text
);
SELECT "weight_kg" FROM "1996_97_season_as_a_three_year_old" WHERE "jockey"='B. York';