CREATE TABLE table_24931 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT, "U.S. viewers  ( million ) " TEXT)

Which series number had 1.98 million viewers?
SELECT MIN("No. in series") FROM table_24931 WHERE "U.S. viewers (million)" = '1.98'
SELECT MIN("No. in series") FROM "table_24931" WHERE "U.S. viewers (million)" = '1.98'