CREATE TABLE table_name_33 (call_sign VARCHAR, frequency_mhz VARCHAR)

What is the name of call sign that uses a Frequency MHz of 91.1 fm?
SELECT call_sign FROM table_name_33 WHERE frequency_mhz = "91.1 fm"
SELECT "call_sign" FROM "table_name_33" WHERE "91.1 fm" = "frequency_mhz"