Table: tv_episode_data_72
Columns: no_in_series
no_in_season
title
directed_by
written_by
original_airdate
u_s_viewers_millions

Question: What number episode in the series was watched by 15.89 million U.S. viewers? 
SQL: SELECT "no_in_series" FROM tv_episode_data_72 WHERE "u_s_viewers_millions" = '15.89'

Question: What is the title of the episode directed by William Webb? 
SQL: SELECT "title" FROM tv_episode_data_72 WHERE "directed_by" = 'William Webb'

Question: How many millions of U.S. viewers watched the episode titled "Once a Hero"?
SQL: SELECT "u_s_viewers_millions" FROM tv_episode_data_72 WHERE "title" = '"Once a Hero"'

Question: Who wrote the episode titled "In the Dark"?
SQL: SELECT "written_by" FROM tv_episode_data_72 WHERE "title" = '"In the Dark"'