what rank has viewers at 6.45?
CREATE TABLE table_15681686_4 (
    rank__week_ VARCHAR,
    viewers__millions_ VARCHAR
)
SELECT rank__week_ FROM table_15681686_4 WHERE viewers__millions_ = "6.45"
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 rank has viewers at 6.45? ### Input: CREATE TABLE table_15681686_4 (
    rank__week_ VARCHAR,
    viewers__millions_ VARCHAR
) ### Response: SELECT rank__week_ FROM table_15681686_4 WHERE viewers__millions_ = "6.45"