What is the 'season joined league' for the club that had 15 seasons at this level?
CREATE TABLE table_61488 (
    "Clubs" text,
    "Seasons at this level" text,
    "Settlements" text,
    "Season joined league" text,
    "Position in 1959\u20131960" text
)
SELECT "Season joined league" FROM table_61488 WHERE "Seasons at this level" = '15 seasons'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: What is the 'season joined league' for the club that had 15 seasons at this level? ### Input: CREATE TABLE table_61488 (
    "Clubs" text,
    "Seasons at this level" text,
    "Settlements" text,
    "Season joined league" text,
    "Position in 1959\u20131960" text
) ### Response: SELECT "Season joined league" FROM table_61488 WHERE "Seasons at this level" = '15 seasons'