CREATE TABLE table_15721 ("Year" FLOAT, "Entrant" CLOB, "Chassis" CLOB, "Engine" CLOB, "Points" FLOAT)

What year was there a lotus 25 Chassis?
SELECT "Year" FROM table_15721 WHERE "Chassis" = 'lotus 25'
SELECT "Year" FROM "table_15721" WHERE "Chassis" = 'lotus 25'