Table schema:
CREATE TABLE table_name_24 (call_sign VARCHAR, erp_w VARCHAR, class VARCHAR, frequency_mhz VARCHAR)

Question: Class of d, and a Frequency MHz smaller than 107.7, and a ERP W smaller than 232 has what call sign?
SQL: SELECT call_sign FROM table_name_24 WHERE class = "d" AND frequency_mhz < 107.7 AND erp_w < 232