CREATE TABLE table_29154676_1 (season_no INT, us_viewers__millions_ VARCHAR)

What is the last episode in the season that had 3.91 million viewers in the US?
SELECT MIN(season_no) FROM table_29154676_1 WHERE us_viewers__millions_ = "3.91"
SELECT MIN("season_no") FROM "table_29154676_1" WHERE "3.91" = "us_viewers__millions_"