Table schema:
CREATE TABLE table_28019988_5 (season__number INTEGER, us_viewers__million_ VARCHAR, written_by VARCHAR)

Question: What episode number of the season was written by Tom Garrigus and had 2.31 million u.s. viewers?
SQL: SELECT MAX(season__number) FROM table_28019988_5 WHERE us_viewers__million_ = "2.31" AND written_by = "Tom Garrigus"