Who wrote the episode where the original air date is july20,2007?
CREATE TABLE "table1_25716399_1" (
  "series_no" real,
  "no_in_season" real,
  "title" text,
  "written_by" text,
  "directed_by" text,
  "original_air_date" text,
  "u_s_viewers_millions" text
);
SELECT "written_by" FROM "table1_25716399_1" WHERE "original_air_date"='July20,2007';