CREATE TABLE table_30975 ("Season no." FLOAT, "Series no." FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" FLOAT, "U.S. viewers  ( millions ) " TEXT)

What is the last episode in the season that had 3.91 million viewers in the US?
SELECT MIN("Season no.") FROM table_30975 WHERE "U.S. viewers (millions)" = '3.91'
SELECT MIN("Season no.") FROM "table_30975" WHERE "U.S. viewers (millions)" = '3.91'