CREATE TABLE table_18080 ("Complete Series" TEXT, "Region 1" TEXT, "Region 2" TEXT, "Region 4" TEXT, "DVD Extras and Bonus Features" TEXT, "Number Of Discs" FLOAT)

What is the region 1 where region 4 is tba is the complete eighth series?
SELECT "Region 1" FROM table_18080 WHERE "Region 4" = 'TBA' AND "Complete Series" = 'The Complete Eighth Series'
SELECT "Region 1" FROM "table_18080" WHERE "Complete Series" = 'The Complete Eighth Series' AND "Region 4" = 'TBA'