CREATE TABLE table_78475 ("Week" FLOAT, "Date" TEXT, "Opponent" TEXT, "Result" TEXT, "Game site" TEXT, "Attendance" TEXT)

Which Opponent was played on 1991-10-13?
SELECT "Opponent" FROM table_78475 WHERE "Date" = '1991-10-13'
SELECT "Opponent" FROM "table_78475" WHERE "Date" = '1991-10-13'