Which Airing date has a Number of episodes larger than 20, and a Genre of modern drama?
CREATE TABLE "first_line_series" (
  "airing_date" text,
  "english_title_chinese_title" text,
  "number_of_episodes" real,
  "theme_song_t_sub_theme_song_st" text,
  "genre" text
);
SELECT "airing_date" FROM "first_line_series" WHERE "number_of_episodes">20 AND "genre"='modern drama';