CREATE TABLE table_16301 ("#" FLOAT, "Air Date" CLOB, "Challenge" CLOB, "Winner" CLOB, "Test-taker" CLOB, "Passed?" CLOB)

What episode aired on 18 April 2007?
SELECT MIN("#") FROM table_16301 WHERE "Air Date" = '18 April 2007'
SELECT MIN("#") FROM "table_16301" WHERE "Air Date" = '18 April 2007'