user: Question: Who directed episode 21?
Columns: ['No. by series', 'No. by season', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production number', 'U.S. viewers (in millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: [59.0, 1.0, '"Rise"', 'Rob Bowman', 'Andrew W. Marlowe', 'September19,2011', 401.0, '13.28']
SQL:

assistant: SELECT "Directed by" FROM "Table" WHERE "No. by season" = 21.0;