What was the season number for production code E2110?
CREATE TABLE "table1_21781578_2" (
  "season_no" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_millions" text
);
SELECT "season_no" FROM "table1_21781578_2" WHERE "production_code"='E2110';