What year was number 7 built?
CREATE TABLE table_15988 (
    "Class" text,
    "Operator" text,
    "No. Built" real,
    "Year Built" text,
    "Cars per Set" real,
    "Unit nos." text
)
SELECT "Year Built" FROM table_15988 WHERE "No. Built" = '7'
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 year was number 7 built? ### Input: CREATE TABLE table_15988 (
    "Class" text,
    "Operator" text,
    "No. Built" real,
    "Year Built" text,
    "Cars per Set" real,
    "Unit nos." text
) ### Response: SELECT "Year Built" FROM table_15988 WHERE "No. Built" = '7'