CREATE TABLE table_21550897_1 (production_code INT, us_viewers__million_ VARCHAR)

What is the production code for the episode that had 9.18 million viewers (U.S.)?
SELECT MAX(production_code) FROM table_21550897_1 WHERE us_viewers__million_ = "9.18"
SELECT MAX("production_code") FROM "table_21550897_1" WHERE "9.18" = "us_viewers__million_"