user: Question: What is the last episode in the season that had 3.91 million viewers in the US?
Columns: ['Season no.', 'Series no.', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: [1.0, 188.0, '"Rispetto"', 'Jean de Segonzac', 'Rick Eid', 'May1,2011', 10004.0, '5.10']
SQL:

assistant: SELECT "Season no." FROM "Table" WHERE "U.S. viewers (millions)" = '3.91';