Table schema:
CREATE TABLE table_name_20 (channel_4_weekly_rank_a INTEGER, viewers__millions_ VARCHAR, airdate VARCHAR)

Question: What is the average Channel 4 weekly rank for less than 2.19 million viewers on December 7, 2007?
SQL: SELECT AVG(channel_4_weekly_rank_a) FROM table_name_20 WHERE viewers__millions_ < 2.19 AND airdate = "december 7, 2007"