CREATE TABLE table_70421 ("Year" FLOAT, "Entrant" TEXT, "Class" TEXT, "Chassis" TEXT, "Engine" TEXT, "Rank" TEXT, "Points" TEXT)

Which class has a year prior to 2011 and audi r15 tdi as the chassis?
SELECT "Class" FROM table_70421 WHERE "Year" < '2011' AND "Chassis" = 'audi r15 tdi'
SELECT "Class" FROM "table_70421" WHERE "Chassis" = 'audi r15 tdi' AND "Year" < '2011'