What is the Segment A entry for the episode that has an entry of Diesel filters for Segment D?
CREATE TABLE "season_21_2013" (
  "series_ep" text,
  "episode" real,
  "segment_a" text,
  "segment_b" text,
  "segment_c" text,
  "segment_d" text
);
SELECT "segment_a" FROM "season_21_2013" WHERE "segment_d"='diesel filters';