CREATE TABLE table_25246990_5 (original_us_air_date VARCHAR2, us_viewers__millions_ VARCHAR2)

How many different original U.S. air dates appear where the U.S. viewers were 2.5 million?
SELECT COUNT(original_us_air_date) FROM table_25246990_5 WHERE us_viewers__millions_ = "2.5"
SELECT COUNT("original_us_air_date") FROM "table_25246990_5" WHERE "2.5" = "us_viewers__millions_"