CREATE TABLE table_203_28 (id DECIMAL, "year" DECIMAL, "competition" TEXT, "venue" TEXT, "position" TEXT, "event" TEXT, "notes" TEXT)

what was the only year that a 5 km event took place ?
SELECT "year" FROM table_203_28 WHERE "event" = '5 km'
SELECT "year" FROM "table_203_28" WHERE "event" = '5 km'